First and foremost, I want to point out that this entire thread is a suggestion and does not require any commitment towards implementing or doing anything about it. The reason I’m adding this disclaimer is because there was a lot of time spent on making this skill but it was entirely born out of my desire and enjoyment to make a personal idea “come to life,” so to speak. I’d like for this topic to be seriously discussed, of course, but in no way am I making demands or attempting to force the devs to implement my ideas! 
Alright, on to the actual suggestion.
A savant of the art of runes, able to infuse magic into offensive and protective wards that devastate those who trespass them.
The Runemaster is a mage whose power comes from the ability to cast runes and either deal damage or defend themselves based on those runes’ capabilities. Certain runes are harder to memorize or recall, but as the mage studies and becomes more familiar with the runes, the power of using them becomes apparent. An expert Runemaster is a sight to behold, even to other mages who may understand the potential within runes.
The ability Lightrune is an intermediate rune for Runemasters, requiring more skill than a basic Rune of Refinement, but becomes more advanced as it is specialized, allowing for massive amounts of destruction or the ability to devastate enemies from far away before they can recognize what is coming. At its core, Lightrune creates a rune on the ground at a targeted location which strikes down with a lightning bolt after a small delay and deals damage to any enemies that are within the area where the rune was created. What makes Lightrune so powerful is that it will also set off other Lightrunes that are placed near the first Lightrune cast when it strikes, which can cause a chain reaction of lightning strikes dealing substantial damage. The ability is simplistic but can become immensely powerful in the hands of an expert.
Lightrune
Sets a rune at the targeted location, which strikes down with lightning damage in an area after X seconds. If other runes are within the area of the strike, they are set off in a chain reaction of lightning strikes.
Mana cost: 15 - 20
Cooldown: 0
Area size: see Osprey Lightmage ability
Base cast speed: low
Tags: Lightning, Spell, Intelligence
Runemaster exclusive ability
X in this case is the amount of time between initially casting Lightrune at the location, and the moment it strikes down with lightning. I wasn’t sure how long it should’ve been, but later on (in the skill tree section) I’ll go into an explanation for two potential amounts of time.
Initially, I think a mana cost of something between 15 and 20 works best because you shouldn’t be able to cast a ton of Lightrunes early on without either investing into mana on your character or choosing specific nodes in the skill tree, which has some mana efficiency and mana gain nodes. Obviously this is adjustable but I think if the mana cost is too low then it stops being a factor in the skill at all (which I think is bad), and if the mana cost is too high then there’s a massive downside where you’re casting a high cost skill, draining most/almost all of your mana, but then not being able to achieve the chain reaction component until you’ve invested further into gear/passives/skill tree nodes, at which point you’ve already spent a large amount of time playing Lightrune without it.
Lightrune not having a cooldown is for the same reason its mana cost isn’t too high. It’s important to allow the player to explore the chain reaction of the lightning strikes to see its potential. Also, if the player so desires, there’s an option on the skill tree that changes Lightrune to be instant cast in exchange for having a cooldown instead.
The area of Lightrune would be very similar to the area that the Osprey Lightmage enemies cast (this is also where I derived a lot of the imagery around Lightrune). This makes it big enough to allow the lightning strikes to overlap in size, which then allows for the chain reactions, but not so big that you could create an entire screen-wide chain of Lightrunes to clear everything on screen.
I’m not actually sure if spell skills have varying “base cast speeds” but if it’s possible, I think Lightrune should have a slower base cast speed to make it feel like the Runemaster is actually getting better at using Lightrune as it levels up in the skill tree. Heavy investment in cast speed allows for faster gameplay with this skill, but if the base cast speed is already high then this type of investment could feel less impactful.
The tags are what you’d typically expect of a spell skill, and Lightrune scales with intelligence like most (all?) of the other mage skills.
Lightrune seems like the type of skill that should be exclusive to Runemasters, since neither Sorcerers or Spellblades (currently) have any rune abilities (which could change), so it would best fit being either a Level 25 or 30 skill on the Runemaster passive tree, or alternatively being the Mastery skill for Runemasters (i.e. the skill you receive for choosing to specialize into being a Runemaster).
This is the skill tree for Lightrune. (Excuse the poor art skills, I only know copy paste and I gave it my best 
and before anyone asks, yes, that is Raigeki) I’m including two other skill trees in the spoilers below because they’ll help explain the nodes that I’m talking about on the skill tree without having to guess which node is located where.
Outline version
Shorthand version
This is the outline that I used to describe each of the nodes and how they connect to each other. You can see where each of these nodes is located on the Outline version of the skill tree. Each node is described with its outline number (1.0 or 2.2.1), then the number of prerequisite points required to reach the node ([0/0] for the points connected to the center, [3/5] requiring 3 points out of a total of 5 points from the node before it), a description of the node (all Xs are arbitrary numbers, they are not all the same number), and then the total number of points that can be allocated into that node (anywhere from 1 pt to 6 pts). If this is confusing, don’t worry, there will be an explanation for each of these nodes and how they connect further down.
1.0 [0/0] +X% increased damage per point (5 pts)
1.1 [2/5] X% chance to apply a Spark Charge on hit per point (4 pts)
1.1.1 [3/4] Detonate Spark Charges if enemy is already affected (1 pt)
1.2 [2/5] X% Shock Chance per point (5 pts)
1.2.1 [3/5] Shocking an enemy with Lightrune grants X mana (2 pts)
1.3 [4/5] +X% increased damage per point, -X% Area per point (3 pts)
1.3.1 [3/3] * (1 or 2 pts, add'l pt grants add'l # of strikes)
2.0 [0/0] +X% increased Cast speed per point (4 pts)
2.1 [4/4] Lightrune strikes quicker (after X sec), but costs X% more mana (1 pt)
2.2 [1/4] +X% Area per point (3 pts)
2.2.1 [3/3] ** (1 pt)
3.0 [0/0] +X% Mana Efficiency per point (4 pts)
3.1 [1/4] +X% Area per point (3 pts)
3.1.1 [3/3] ** (1 pt)
3.2 [4/4] Lightrune is now instant cast, but has an X second cooldown (1 pt)
3.2.1 [1/1] X% increased cooldown recovery speed (3 pts)
3.2.2 [1/1] Lightrune can store an additional charge (1 pt)
4.0 [0/0] X% increased Critical Strike chance (5 pts)
4.1 [2/5] +X% increased cast speed if Lightrune has Crit recently (2 pts)
4.2 [4/5] Lightrune has +X% critical strike chance (4 pts)
4.2.1 [3/4] +X% Critical Multiplier per Lightrune strike in chain reaction (1 pt)
5.0 [0/0] +X Ward on hit (6 pts)
The explanations for each of these nodes is under each of these spoilers, which are marked with both their outline number as well as the wording in the Shorthand skill tree (see above for those skill trees) and will list all the relevant information for each node.
1.0 (inc dmg)
5 points. No prerequisites
Damage: +X% per point
Generic damage increase, nothing special with this node.
1.1 (spark charges)
4 points. Requires 2 points in 1.0
Spark Charge Chance: +X% per point
Lightrune can apply Spark Charges similar to Lightning Blast or Elemental Nova.
1.1.1 (detonate spark charges)
1 point. Requires 3 points in 1.1
If Lightrune hits an enemy already affected by Spark Charge the Spark Charge explodes, dealing lightning damage to all nearby enemies.
Detonating Spark Charges allows for more bursty area damage when Lightrune hits multiple enemies.
1.2 (shock chance)
5 points. Requires 2 points in 1.0
Shock Chance: +X% per point
Increased shock chance for the purposes of 1.2.1's mana gain node. Also technically a damage increase from shock innately.
1.2.1 (mana on shock)
2 points. Requires 3 points in 1.2
You gain X (per point) mana for each time an enemy is Shocked with Lightrune
Each shock inflicted would count as a mana gain, so having over 100% chance to shock could potentially grant twice as much mana gain. This would probably need a maximum limit if X is too high. Multiple points in this node would increase the amount of mana gained on shock.
1.3 (inc dmg, dec area)
3 points. Requires 4 points from 1.0
Damage: +X% per point
Area: -X% per point
Increases the damage of Lightrune at the cost of decreasing the size of its area, which makes it harder to cause chains of Lightrune strikes. The increase in damage would have to be significant enough to make the decrease in area worthwhile.
1.3.1 (* multistrike)
1 or 2 points. Requires 3 points in 1.3
Lightrune no longer causes other casts of Lightrune to strike when it strikes; instead, each cast of Lightrune strikes in the same area X (per point) additional times. This node is incompatible with 2.2.1. If you have taken both, this will not work.
First thing to note, is that this node is incompatible with 2.2.1 because they both remove the ability for Lightrune to chain react, and both being compatible would make players feel compelled to pick up both nodes to receive the most amount of power in exchange for removing the potential power of chain reactions from the skill itself, which becomes a forced choice which isn't meaningful or exciting for the player anyway. Also I'm not sure how you could have it such that multiple Lightrune casts could strike at the same time (2.2.1) but then also repeat the same strikes afterwards (1.3.1)? Either way, these two nodes are incompatible for now.
This node works by removing the ability for Lightrune to chain react, such that any additional casts of Lightrune would work entirely separate of the original cast of Lightrune. For example, if Lightrune takes 1 second to strike after cast, and the player casts Lightrune A at t=0 seconds and then casts Lightrune B at t=0.7 seconds (even if Lightrune B is within the area of Lightrune A), then A will strike at t=1 second and B will strike at t=1.7 seconds. With the additional strikes, what happens is Lightrune will strike its area and then strike the same area X more times immediately afterwards. So, referring back to the example, if X (the number of additional strikes) is 2 and you allocated 1 point in this node, then Lightrune A would strike 3 times at t=1 seconds and Lightrune B would strike 3 times at t=1.7 seconds.
Functionally, damage-wise this is no different than just having a 200% or 300% increased damage node, or a 100% chance to deal double damage node, but it gives it a "multistrike"-like feel to the skill, hence the shorthand name.
Also, I wanted to add that this node could have 1 or 2 points in it, if you wanted the player to have to invest more points to get more damage (e.g., if X is 1 but the skill has 2 points maximum, which would require more point investment than if X were 2 but the skill had 1 point maximum).
2.0 (cast speed)
4 points. No prerequisites
Cast Speed: +X% per point
Generic increase in cast speed, although I think this could have a value slightly higher than average to feel like the Runemaster is improving with Lightrune itself over time, and to offset the lower base cast speed as mentioned above.
2.1 (-delay +mana)
1 point. Requires 4 points from 2.0
Mana Cost: +X
The amount of time until Lightrune strikes after casting is (increased/decreased) by X seconds.
This node is the reason why I had some difficulty figuring out how long I wanted Lightrune to take until activating its lightning strike. With a longer delay between casting and activation, there is more time to create large chains of strikes with significant investment in cast speed and mana efficiency or a large mana pool (hence the increased mana cost), which has synergy with 4.2.1; however, with a shorter delay between casting and activation, there is better precision (enemies tend to move away if the delay is long) and there's decent synergy with 1.3.1 because you wouldn't have to care about not having enough time to chain react if there weren't any chain reactions.
My original thoughts were that Lightrune would have a longer delay (something between 1.5 and 2 seconds) and this node would halve that delay (so 0.75 to 1 seconds) in exchange for a much larger mana cost. Now I have the opposite view, where it has a smaller delay so the player can't initially start off doing massive chains, and instead must invest in a longer delay to achieve larger chains, although in that case the increase in mana cost would be a lot smaller. The latter case makes more sense in that the player has to invest into the skill (akin to improving with the rune) in order to get more value out of it, rather than having, in the former case, a node that essentially provides an option for better quality of life in exchange for potential power.
This is definitely the biggest conflict that I've faced, because both of these two scenarios are valid and it comes down to an arbitrary choice, but there isn't really a solution where you can pick both since it also affects how the base skill works even without this node.
2.2 (inc area)
3 points. Requires 1 point from 2.0
Area: +X% per point
This node 2.2 is the same node as 3.1 as seen in the skill trees. They have different pathing which is why they have different outline numbers, but are actually the same node.
Increases the size of the area of Lightrune which can help achieve longer chain reactions by covering a larger amount of area. Generally a good node to increase clearspeed.
Ideally, the increase in size from this node would be smaller than the decrease in size from 1.3 so that if someone wanted to take 1.3.1 they couldn't offset the smaller area size by simply taking this node too. There is an argument where they have the same value so they cancel each other out, but I think the damage increase from 1.3 should be significant enough such that the downside isn't so easily undone.
2.2.1 (** all strikes at same time)
1 point. Requires 3 points in 2.2
Lightrune no longer causes other casts of Lightrune to strike when it strikes; instead, all casts of Lightrune after the initial cast will strike at the same time as the initial cast's strike. This node is incompatible with 1.3.1. If you have taken both, 1.3.1 will not work.
First thing to note is that 2.2.1 is the same as 3.1.1, these are the same node as seen in the skill trees, just with different ways to get to them, hence the different outline numbers.
Second thing to note, is that this node is incompatible with 1.3.1 as was explained there. Essentially, they both remove the ability for Lightrune to chain react, so both being compatible would mean the player is forced to take both if either is wanted.
This node works by removing the ability for Lightrune to chain react, by instead causing all casts of Lightrune after the initial cast to strike at the same time. The difference here is that the chain reactions are caused by the initial area overlapping a later area such that the initial area will strike first, and then the overlapping areas strike quickly afterward, and then any overlapping areas for that strike will then strike themselves, and so on. With this node, the chain reactions are removed and instead the strikes, regardless of where they are placed, all occur at the exact time the first strike occurs. For example, without this node, if the amount of time it takes for Lightrune to strike after being cast is 1 second, and the player casts three Lightrunes in a line slightly overlapping each other within that 1 second, then the first cast will strike, and then quickly after the second will strike and then the third will strike. This example changes when taking this node, such that not only can the Lightrune casts be far away from each other (non-overlapping), but also as long as they're all placed within the 1 second window, they will all strike down at the same time, specifically at 1 second after the first Lightrune is cast.
An important effect is that with this node, any Lightrune casts that occur outside of the window of time between the initial Lightrune cast and its strike will start a separate window of time which would function identically to any other initial Lightrune cast. This is because the first window of time will already have completed its strikes and dealt its damage, so the first cast that falls outside of this window would have to begin a new, separate window, which would be equivalent to starting a new, initial cast of Lightrune.
3.0 (mana effic)
4 points. No prerequisites
Mana Efficiency: +X% per point
General mana efficiency node, helpful if you're taking 2.1 and want to offset the increased mana cost, but also great if you're trying to do a super-fast-cast-speed build with tons of chain reactions.
3.1 (inc area)
See 2.2 (inc area) above.
3.1.1 (** all strikes at same time)
See 2.2.1 (** all strikes at same time) above.
3.2 (cooldown, instant cast)
1 point. Requires 4 points in 3.0
Lightrune is now instant cast, but has an X second cooldown.
Introduces a cooldown to Lightrune to offset the massive benefit of being instant cast. This cooldown would have to still be pretty short (or if not, then 3.2.1 should have a large amount of cooldown recovery speed) because Lightrune is designed to be used on top of and connected with itself. At the very least, it wouldn't make sense to have this cooldown be longer than the time it takes for Lightrune to strike.
3.2.1 (cooldown recovery)
3 points. Requires 1 point in 3.2
Cooldown Recovery Speed: +X% per point
This could either be a small amount of cooldown recovery speed to make using Lightrune feel smoother (assuming the introduced cooldown isn't too long), or this could be a significant amount of cooldown recovery speed (in the hundreds of %) as a way to offset the significant cooldown that was introduced in 3.2. Either way, this node is either a requirement or a nice quality of life option for choosing the instant cast path.
3.2.2 (+ skill charges)
1 point. Requires 1 point in 3.2
Lightrune can store an additional charge, allowing you to use it twice before it goes on cooldown.
This node "changes" Lightrune from a simple cooldown skill to a charge-based skill, although I put changes in quotes because it's not really a significant change. The added benefit to having 2 charges is that you can store up while moving between enemy groups, and it also smooths out the annoyance of having to wait for a cooldown if it's too long. I assume that it would be difficult to balance this node around being able to get more than one charge, since too many charges combined with the cooldown recovery speed would make this the default path for this skill, which would be a forced choice.
4.0 (inc crit chance)
5 points. No prerequisites
Increased Critical Strike Chance: +X% per point
Note that this is supposed to be increased critical strike chance, and not base critical chance, which is what 4.2 gives instead. Otherwise, this is a generic critical strike chance increase.
4.1 (cast speed if crit recently)
2 points. Requires 2 points in 4.0
X% increased Cast Speed if you have landed a critical strike with Lightrune recently (in the past 4 seconds).
This node is really interesting, since it gives you multiple ways to play with it. For one, you can use it as a slight cast speed boost every once in a while if you don't want to spend too many points or equipment to invest in critical strikes. For two, the cast speed isn't exclusive to Lightrune which means you can technically use Lightrune as utility. Lastly, if you do decide to invest heavily in critical strikes (and there's already a huge benefit to doing so), then you can essentially have a permanent cast speed buff! This is definitely one of the more unique nodes.
4.2 (+% base crit chance)
4 points. Requires 4 points in 4.0
Base Critical Chance: +X% per point
Note that this is base critical chance, which increases the base chance to critical strike with this spell, unlike 4.0 which is increased critical strike chance. I think overall this node should be stronger than 4.0 but that depends entirely on how the player is gearing, so either 4.0 or this node could probably be improved in some way...
4.2.1 (+crit multi with higher chains)
1 point. Requires 3 points in 4.2
Lightrune gains X% additional Critical Multiplier for each additional strike within a chain reaction of strikes. If chain reactions are disabled by 1.3.1 or 2.2.1, the amount of Critical Multiplier is a fixed amount.
I'm splitting this into three parts: the first part will explain how the Critical Multiplier works when neither 1.3.1 nor 2.2.1 are chosen. The second part will be the explanation with 1.3.1 chosen, and the third part will be the explanation with 2.2.1 chosen.
Neither 1.3.1 nor 2.2.1
In this case, the chain reactions component of Lightrune is enabled. Each chain of strikes is determined by the number of Lightrune casts with overlapping areas before the first Lightrune cast strikes, at which point the chain reaction begins and all of the overlapping Lightrunes strike in order of how their areas overlap. For simplicity, we'll define the number of chains as the total number of strikes that end up occurring in a chain reaction. By this, a single cast of Lightrune is technically a chain of 1. A chain of 2 would be two Lightrune casts where their areas overlap, and the first cast setting off the second cast before it would have "normally" occurred. A chain of 3 would be 3 Lightrunes, etc. The amount of additional Critical Multiplier (for all hits within the chain reaction, so the chain count would have to be determined before damage is dealt) would be 0 for a chain of 1, X for a chain of 2, 2X for a chain of 3, 3X for a chain of 4, etc. Essentially the number of additional strikes in a chain is the amount of additional Critical Multiplier added to the chain reaction.
1.3.1
In this case, chain reactions are removed in exchange for each individual cast of Lightrune being able to hit multiple times in the same location. The amount of Critical Multiplier would be determined by the fixed amount of strikes that occurs on each cast of Lightrune. For example, if Lightrune only strikes once per cast (this hypothetical isn't possible but used for explanation), then there would be 0 additional Critical Multiplier. If Lightrune strikes twice per cast (i.e, one additional strike from 1.3.1), then there would be X additional Critical Multiplier. If Lightrune strikes three times per cast (i.e, two additional strikes from 1.3.1), then there would be 2X additional Critical Multiplier. The number of additional strikes per individual cast of Lightrune determines the amount of additional Critical Multiplier.
2.2.1
In this case, chain reactions are removed in exchange for all Lightrune casts hitting at the same time after the initial cast. The amount of additional Critical Multiplier would be a fixed flat amount, probably around a third the amount compared to the amount of Critical Multiplier given if 1.3.1 were active with this node. (Subject to balancing, of course.)
5.0 (ward on hit)
6 points. No prerequisites
Ward On Hit: +X per point
Lightrune hits grant you ward. Simple defensive component if desired, and with 6 points this can scale up to a high amount of ward gain on hit.
I’d like to thank anyone who reads through the entirety of this suggestion, and if even a small component of this goes towards helping develop Last Epoch or inspiring anyone at all then I’m beyond satisfied! It’s been fun thinking of ideas and coming up with potential connections and writing it all up in the end. 