Undying is a keyword triggered ability introduced in Dark Ascension  and also featured in Avacyn Restored. Its function is similar to persist, a keyword ability from the Shadowmoor block; if a permanent with undying is put into a graveyard from the battlefield, it returns to the battlefield under its owner's control. However, unlike persist, a creature with undying returns to the battlefield with a +1/+1 counter on it (rather than a -1/-1 counter) unless it already had one or more of those counters on it when it died.
From the Comprehensive Rules
- 702.92. Undying
- 702.92a 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."
- All cards printed with undying are creatures. If a creature with undying stops being a creature (for example, an artifact creature with undying becomes a noncreature artifact due to Neurok Transmuter's second ability), 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.
- Sightless Ghoul -
Creature — Zombie Soldier
Sightless Ghoul can't block.
Undying (When this creature dies, 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.)
There are 20 cards that involve undying, which divide by color as such:
- Black = 45 %
- Red = 20 %
- Green = 20 %
- Blue = 15 %
Spell that grants Undying