Honestly DH would be fine if it didn't proc so easily.

What I mean is, fucking red buff, scorch... hell even pets, ignite, and poisons can proc DH

I think DH is absolutely fine and making it proc on abilities is perfectly fine, but there should be a threshold inorder to proc the rune.

Something like "attacks that would deal 50-100 damage against a target at 50% health will deal extra damage and harvest a soul essence"

or/alongside "third party damage sources (items, runes, buffs, ally afflicted bonuses) do not apply dark harvest. "

Would ton down how easy stacking it is, reducing the frequency of crazy stack numbers, and it wouldn't completely outclass other runes with certain kits (been playing DH mages like morgana and ziggs and its fucking stupid how you can just throw undodgeable spells like tormented soil for free extra damage and permanant scaling damage)