Any time a character takes
damage from a single hit that exceeds the character's massive
damage threshold, that
damage is considered massive
damage. A character's massive
damage threshold is equal to the character's current Constitution score; it can be increased by taking the
Improved Damage Threshold feat.
When a character takes massive
damage that doesn't reduce his or her
hit points to 0 or lower, the character must make a Fortitude save (DC 15). If the character fails the save, the character's
hit point total is immediately reduced to -1. If the save succeeds, the character suffers no ill effect beyond the loss of
hit points.