Artifact, 5 (5) Whenever a creature is put into a graveyard from the battlefield, if another creature is on the battlefield, return the first card to the battlefield under its owner's control at the beginning of the next end step.1
Illus. D. Alexander Gregory
10/4/2004: If more than one creature is on the battlefield and all the creatures on the battlefield go to the graveyard at once, then all of them are returned at end of turn. This is because all "leaves the battlefield" triggers that check the state of the game do so at the time right before the card left the battlefield.
10/4/2004: It checks to see if there are any creatures on the battlefield controlled by any player at the time the creature is put into the graveyard. If there are, it will put the creature back even if there are no creatures on the battlefield at the end of turn.
10/4/2004: It fails to bring the creature back if the creature is not still in the graveyard at the end of turn.
10/4/2004: It works for all players and has errata to remove the "your graveyard" text.
10/4/2004: The creature comes back even if Lifeline leaves the battlefield after triggering, but before it resolves.
10/4/2004: If multiple creatures are coming back, they come back one at a time, not all at once. This is because Lifeline triggered once for each creature and set up a separate "at end of turn" effect for each.
Urza's Saga
(Rare)