When you would reduce a creature to 0 Hit Points with a melee attack, you can instead reduce the creature to 1 Hit Point. The creature then has the Unconscious condition and starts a Short Rest.
The creature remains Unconscious until it regains any Hit Points or until someone uses an action to administer first aid to it, which requires a successful DC 10 Wisdom (Medicine) check.
{
"name": "Knocking Out a Creature",
"page": 370,
"srd52": true,
"__prop": "variantrule",
"source": "XPHB",
"entries": [
"When you would reduce a creature to 0 {@variantrule Hit Points|XPHB} with a melee attack, you can instead reduce the creature to 1 Hit Point. The creature then has the {@condition Unconscious|XPHB} condition and starts a {@variantrule Short Rest|XPHB}.",
"The creature remains Unconscious until it regains any {@variantrule Hit Points|XPHB} or until someone uses an action to administer first aid to it, which requires a successful {@dc 10} Wisdom ({@skill Medicine|XPHB}) check."
],
"ruleType": "C",
"basicRules2024": true
}