Whenever a creature dies, 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.
If more than one creature is on the battlefield and all the creatures on the battlefield go to the graveyard at once, then none of them are returned at end of turn. This is because Lifeline’s ability has an intervening-if clause, which means that there must be at least one creature on the battlefield at the time the ability resolves.