Variant Rule: Massive Damage

This rule is found in the Dungeon Master's Guide (2014), p. 273, though it is used here with some alterations to make it less punishing for low-level characters.
When a creature takes damage from a single source equal to or greater than half its hit point maximum, it must make a DC15 Constitution saving throw or suffer a randomly determined effect on the System Shock table below. For example, a creature with a maximum of 50 hit points would need to make a DC15 Con save if it took 25 damage (or more) from a single source.