Personally I think that a mana equivilent of Survivor's Instinct would be more suitable.
With your idea, how about there being a Stamina cost to absorb Mana? It makes sense to use another resource to replenish the other. Also, there's the problem that the enemies don't have mana. They just keep casting to their hearts desire. This could be fixed of course. Perhaps enemies can have a mana rating, but still have unlimited mana. I think they need unlimited mana otherwise it would be too easy to drain them, making magical enemies defenseless.
