Dawnmourne, City of Rogues
No matter how experienced or skilled a character may be, a knife in the kidneys or an axe to the skull can still bring him down. This vulnerability is reflected in the standard massive damage rule, which sets a level of damage to which any character might succumb. Here, that threshold of damage is much smaller than in the standard rules of the PHB, but characters can still survive major amounts of damage—if they’re lucky.
Massive Damage Treshold
Every character has a massive damage threshold. Whenever a character takes this amount of damage (or more) from a single attack (but not from multiple separate attacks, even if in the same round) he must make a massive damage save.
A character’s massive damage threshold is normally equal to his Constitution score plus Size modifier (Fine –16, Diminutive –12, Tiny –8, Small –4, Medium +0, Large +4, Huge +8, Gargantuan +12, Colossal +16) plus his armor bonus (including magical enhancement) plus his natural armor bonus (if any). The Improved Damage Threshold and Greater Damage Threshold feats increase a character’s massive damage threshold. If the character’s Constitution score increases, so too does his massive damage threshold. Likewise, if his size is increased, his massive damage threshold increases accordingly.
Massive Damage Save
If a character takes damage equal to or greater than his massive damage threshold in a single attack, he must make a massive damage save. This is a DC 15 Fortitude save (some special abilities can increase this DC). A successful save means that the character suffers no additional effects from the attack, other than the loss of hit points.
Effects of Massive Damage
A character who fails his massive damage save is immediately reduced to –1 hp and is dying. He immediately falls unconscious, and will continue to lose hit points unless stabilized (just as any character reduced to negative hit points).
Characters who are reduced to negative hit points are also at increased risk of suffering Severe Injuries or infected wounds