Because as assassins they should get a sabotage ability.
Backwash: curse an enemy with the power of who-the-flip-cares, reversing the effects of all healing attempted by them in order to cause damage to themselves and allies equivalent to 50% of the attempted healing.
Example: I curse a sorc, he casts a 8000 heal on himself and instead takes 4000 damage.
Let’s make it happen!
(This is actually the result of a conversation I had with a friend to invent the most ridiculously toxic ability in the game, and he suggested I post it to make the community get all riled, lol. Obviously it doesn’t work, but I was inspired by how much I used to love slipping poisons into someone’s pocket then watch them die from it. Who does that?? “Well, lookee here, a random green vial with a skull has appeared in my pocket. I should drink it...irk!” Stupid NPC....just be happy I didn’t hit you with the Bone of Mehrunes Dagon, because this is a family game.)