How about Mana Burn?

[Tribulation]

I really don't like the idea, especially since you suggested draining all the mana, that would make cp or base camping easier, which I oppose. I don't like the concept of mana burn for BT, I don't believe it fits into the game. For DotA it's alright, as that game focuses a lot more on a big mana pool and mana regeneration, but BT does not work that way, you don't even have enough mana to cast 2 spells at certain levels (HT's ultimate and Rocket, for example).