Undying

Undying is a keyword triggered ability introduced in Dark Ascension. 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 ()

Rulings

• All cards printed with undying are creatures. If a creature with undying stops being a creature (for example, if it's an animated land), undying will still trigger.
• 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 have undying. However, if a token with undying is put into a graveyard, like all tokens, it can't return to the battlefield and will cease to exist the next time state-based actions are checked.
• 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.