Undying

Undying is a keyword triggered ability introduced in Dark Ascension.

From the Comprehensive Rules (November 8, 2024—Edge of Eternities)

• 702.93. Undying
  • 702.93a Undying is a triggered ability. “Undying” means “When this permanent is put into a graveyard from the battlefield, if it had no +1/+1 counters on it, return it to the battlefield under its owner’s control with a +1/+1 counter on it.”
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• All cards printed with undying are creatures. If a creature with undying stops being a creature, undying will still trigger.
• Likewise, if a noncreature permanent becomes a creature, gains undying, and is put into a graveyard, the undying ability will still trigger even though that permanent won’t be a creature when it returns to the battlefield.
• Multiple instances of undying trigger separately, but are effectively redundant as only the first instance to resolve will have any effect.
• An object returned to the battlefield with the undying ability is a new object with no memory of its previous existence. It has “summoning sickness“.
• Tokens can gain undying, and the ability will trigger if a token is put into a graveyard. However, like all tokens, it can’t return to the battlefield and will cease to exist as a state-based action before the ability would go on the stack.
• Undying relies on the last known information about the permanent just before it was put into a graveyard. If that permanent had a +1/+1 counter on it, then receives enough -1/-1 counters for it to be put into a graveyard due to having 0 toughness or lethal damage marked on it, it won’t return to the battlefield despite the state-based action of +1/+1 counters and -1/-1 counters “cancelling each other out”. As the permanent last existed on the battlefield, it had a +1/+1 counter on it, so undying will not trigger.