GURPS Fantasy 53 lists this as a Trigger for Unkillable 3.
Strangely -12% which falls between the -15% for Occasional and the -5% for Common or Very Common
This might be half of the -25% for Rare (rounded "up" toward zero drops the .5) because of how it doesn't completely prevent them from healing and becoming conscious, only from getting their body (and all their other advantages except for Possession and Mind Control, presumably) back.
http://forums.sjgames.com/showthread.php?t=87812 discusses the weirdness