Negative HP

House rule about one's CON determining how far negative a PC can go before death.

At 0 hp combat action action is NOT possible (about to pass out), but can take 1 round for a very simple action for something basic like quaffing a healing potion/herb, etc, (if already out of pack or within easy reach) and then pass out the following round (unless brings them back up to 1 hp or higher, but can take no further action that round).
They are considered "stable" at 0, and do not lose hit points (unless bleeding for other reasons).
Negative hit points to the equivalent of their CON stat. So if CON:13, can go to -13 and still be alive, but at -14 dead. And someone with a CON:6 can go to -6 and still be alive, but dead at -7 or lower.
Once negative, must make successful CON check to stabilize, otherwise once at -1 or lower, lose 1 hp per round (CON check each round to auto stabilize). If someone with healing skills (or magic), succeeds with check, that can also stabilize countdown to death.