Optional Rule: Massive Damage

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.

System Shock

d10 Effect
1 The creature drops to 0 hit points.
2-3 The creature drops to 0 hit points but is stable.
4-5 The creature is stunned until the end of its next turn.
6-7 The creature can't take reactions and has disadvantage on attack rolls and ability checks until the end of its next turn.
8-10 The creature can't take reactions until the end of its next turn.

_{Source: DMG, page 273}