Blood Mana is a curse that is cast by Succubi and Baal. This curse causes damage to you every time you use mana to cast a spell. The damage is equal to the amount of mana that you used up when you cast the spell. This happens even for non-damaging spells, such as Teleport, or Enchant.
This curse is used only when a character's mana exceeds their health. If a character has more health than mana, a curse that halves their defense is cast instead.