Massive damage is a pencil-and-paper rule originally included in NWN but later removed. This rule states that if a character receives at least 50 points of damage in combat, that character will die unless a DC 15 fortitude saving throw is successful. It had been enabled only in the "hardcore" and "very difficult" difficulty settings.
This rule was removed when the Hordes of the Underdark expansion came out (patch 1.59). With the introduction of epic levels, it became rather easy for players to inflict 50 points of damage with a single blow, which made the rule unbalanced (and less fun, according to BioWare's quality assurance department). The current option for warrior types to inflict an instant-death blow is the epic feat devastating critical.