When you proc Spirit Plague using Life’s Journey shield on melee hit while speced into Contagion which increases the mana cost by 25, it doesn’t cost any mana when proced. Not sure if this is intentional.
Also I think Spirit Plague is missing a lot of damage when proceed from melee swing, maybe it’s not carrying over multipliers within the tree? It seems like the damage is cut in half when I melee swing to proc it verse manually cast it.