When you are reduced to 0 Hit Points and not killed outright, you can drop to 1 Hit Point instead, and you regain a number of Hit Points equal to three times your Paladin level. Once you use this feature, you can't do so again until you finish a Long Rest.
Additionally, you can't be aged magically, and you cease visibly aging.
{
"name": "Undying Sentinel",
"page": 116,
"level": 15,
"__prop": "subclassFeature",
"header": 2,
"source": "XPHB",
"entries": [
"When you are reduced to 0 {@variantrule Hit Points|XPHB} and not killed outright, you can drop to 1 {@variantrule Hit Points|XPHB|Hit Point} instead, and you regain a number of {@variantrule Hit Points|XPHB} equal to three times your Paladin level. Once you use this feature, you can't do so again until you finish a {@variantrule Long Rest|XPHB}.",
"Additionally, you can't be aged magically, and you cease visibly aging."
],
"className": "Paladin",
"classSource": "XPHB",
"subclassSource": "XPHB",
"subclassShortName": "Ancients"
}