When using shadow cascade with Coated Blades, the mana correctly increases by 5 per point.
Base Shadow Cascade cost: 15 mana
I tested with 5 points into Coated Blades: +25 mana, for a total of 40 mana.
I then created 4 shadows in town and used the skill and my mana went from 79 down to -29ish.
My cast: 40 MP
Cast per shadow: 25 MP
Total cast with 4 shadows: 140 MP
I do have 2 points in Fight in the Shadows, which would restore 24 mana total with 4 shadows.
Mana pool: 79-140+24 = -37. It happens pretty fast and there’s a slight delay so natural regen might explain why I saw -29 instead of -37.
When I test with no points in Coated Blades, I use 15 mana every time (regardless of how many shadows I have out).
I am not sure if this is intentional or not, but man did it confuse me when I was grinding and kept getting several hundred negative mana!