Skull of Pain

[WhYdOyOuThInKtHaThApPeNeD]

I found it fairly late game. I didn't keep track of the % drop each battle but sometimes it was more than 1%. I got the spell off of it quite fast actually. The boost to rage increase fairly well offset my mana loss too. Combine it with the rage eater, spear of rage and rage sign set.
And it's nice, once the spell is gone you don't have to upgrade it so you can keep the 50% rage boost and don't lose your mana if you prefer that route.