Massive Damage Threshold (Mas)

When a creature takes damage from a single attack equal to or greater than its current Constitution, it must succeed on a Fortitude save (DC 15) or immediately drop to -1 hit points. If the damage would reduce the creature to -1 hit points or fewer anyway, the massive damage threshold does not apply, and the creature does not need to make a Fortitude save.
Constructs, elementals, oozes, plants, and undead ignore the effects of massive damage and do not have massive damage thresholds. Vermin gain a +5 species bonus on their Fortitude saves to avoid falling to -1 hit points.