Blood Ritual

Blood Ritual is a Witch Doctor passive skill, unlocked at level 20.

In-game
This is one of the few methods of actually taking damage that ignores all forms of damage reduction; one could think of this as a reversed Mana Shield.

Given the Witch Doctor's massively small Mana pool compared to their Life pool, the damage suffered from casting, especially at higher levels, is, to put it bluntly, pathetic. This effectively turns the skill to a free 20% Mana cost reduction and 1% Life regeneration.

Even if the wounds from casting somehow manage to overpower the Witch Doctor's regeneration, it still cannot cause them to die, leaving them at 1 point of Life.