Historical Rules · 5 June 2017
Rule 702.92a
Official source text from the 5 June 2017 Rules release
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.”