A couple of [Fighter, Vile] feats, for all your cruelty needs.

[Westhart]

Is there a duration on the -2 penalty?

There is now

Make it just crits and death attacks or add a threshold for sneak attacks to trigger it... something like:

Whenever you score a critical hit, use a death attack, or use a sneak dealing more than X points of extra damage with a weapon you have improved critical for...

Alternatively require a roll of Nat 6 on one (or more depending on how restrictive you want to be) sneak attack die for sneak attack to trigger

EDIT: Gah!! so many ninjas... (what I get for leaving a reply unfinished for a couple hours after I start typing it...)

Just removed sneak attack as that seems to be too powerful for everyone.

shouldn't that be lifespan is now a number of rounds = to their hp past when they run out of healing spells/fast healing?

There should be, but could be pretty long... possibly reduced based on many hit die the victim has and maybe a fort save for half duration... Say 15 minus one-fifth dit hie rounds?

If you're feeling generous give two saves one immediately to cut the duration in half and if that fails a second half way through the duration to end it immediately...

A one hd victim that failed both fort saves takes penalties for 15 rounds; an Ancient Force Dragon (65 hd) that succeeds one of its saves takes a penalty only takes a penalty for 1 round; a Great Wyrm Force Dragon (75 hd) doesn't take a penalty no save required...

Nah, just added a straight one based off of the user's BAB