How about this; instead of a flat hp% decrease, Lifedrinker just deals an amount of shadow damage with the healing component. It should be on par with the other high level spells, though.
I see Quench the Spirit as sort of the logical progression of the Witchcraft school, from slowly weakening the target to simply killing it outright. Perhaps instead of that, it could be something like this:
Quench the Spirit: Deals x-y shadow damage to one opponent. If the opponent dies as a result of the damage, an undead warrior is created from the target's remains within 2 turns.
