Ooh, this looks interesting. +1
I like the 'draining mana' idea, since their mana is already draining and hunting should be able to give them more than just food and settling their cravings. It would let them stay in spider form or whatever longer (as far as I'm assuming).
What I don't like is the mechanic of wolves being the spiders unless they're the large spiders. It wouldn't make sense, and if one has too many wolves/spiders, it might cause lag. Although, I may be wrong, since I remember something about two spiders? I'm not sure, I'm forgetting what I read a few minutes ago (I'm lame).
Maaaaybe draining life force isn't a good idea. You'd have to have fantastic roleplayers to pull that off without seeming.. powergame-y. I mean, I personally wouldn't mind if that happened to me, but others might complain about how they didn't have a choice in what was happening.