Massive Damage

Source: Dungeon Master’s Guide p. 273

This optional rule makes it easier for a creature to be felled by massive damage.

When a creature takes damage from a single source equal to or greater than half its hit point maximum, it must succeed on a DC 15 Constitution saving throw or suffer a random effect determined by a roll on the System Shock table. For example, a creature that has a hit point maximum of 30 must make that Constitution save if it takes 15 damage or more from a single source.