Undying

Undying is a keyword triggered ability introduced in Dark Ascension  and also featured in Avacyn Restored and Modern Horizons.

Description
Undying's 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.

Rulings

 * 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.

Spells that grants Undying

 * Undying Evil
 * Mikaeus, the Unhallowed (Non-Humans)