Template that lets you "Mutate" into a Man-Spider?

[kevin video]

I'd rather not go were-spider if at all possible, but this is more like a take-over. Like in some shows, when someone's slowly being possessed, or if they're slowly mutating, but it's reversible. That's what I'd like to do to a PC that's now an NPC.

I was thinking of just adding the arachnoid template, but that says animal or magical beast. Is there something that could affect a humanoid? I was thinking drider, but that's not really something that's temporary. I guess I could fluff it, but I'd rather the rest of the party recognized the NPC.

Any ideas as to what's available? If arachnoid is something that would work, I can just go with that. This is just a temporary thing anyways. If they succeed in the next adventure, he'll be reverted back to normal.

EDIT: Just found Bebilith-Possessed (Demon-Possessed Creature). That could work too.

[cru]

Look up Spider Curse from Spell Compendium.

[Ithamar]

There is a lesser drider template that basically gives you the lower body of a spider.  It is on the WotC website.  Like +1 or +2 LA as I recall.

[kevin video]

There is a lesser drider template that basically gives you the lower body of a spider.  It is on the WotC website.  Like +1 or +2 LA as I recall.

Do mean this one? http://www.wizards.com/default.asp?x=dnd/mm/20011118a

[Ithamar]

Yes, that appears to be the one.  Not sure about the LA though.  Hmm...