Cultist - 5 mana to summon. 0 armor, 6 health, 1 die attack, Bloodthirsty +1 - Special: Self-Flagellation - Dark Cultist attacks itself and you gain mana equal to the damage Dark Cultist takes.
I think this would be pretty useful because you could move them around unlike mana crystals and slap Regrowth on them to potentially keep them alive indefinitely. Of course, with the Bloodthirsty they may die even with Regrowth; but, I think that just balances out the fact that they could be a much faster source of mana than a crystal.
As I've got it statted you still gain a little extra mana in the long run even without Regrowth, but I'm not sure if it's really worth it that way. To make it more worthwhile perhaps expand the special to say that if Cultist kills itself gain 3 mana.