Heroes 5.5 WarGame Edition - Tactical Edition.

I'm currently considering returning the demons to the racial branch of Hellfire, but removing the "Curse Mark" and giving it to the Dungeon faction as the "Assassin's Mark." From it, I would create "Assassin's Strike" - applying a debuff on hero attack (removing it from the "Attack" skill). Under a developed/skilled Darkness, the damage of the debuffed stack will be approximately halved compared to a stack without the debuff.
The reason is that Hellfire is mana-intensive, and therefore it combines with the Corpse Eater, which demons already have.