Lifeline's current wording is:

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.

The “intervening ‘if’ clause” rule (CR 603.4) says:

A triggered ability may read “When/Whenever/At (trigger event), if (condition), (effect).” When the trigger event occurs, the ability checks whether the stated condition is true. The ability triggers only if it is; otherwise it does nothing. If the ability triggers, it checks the stated condition again as it resolves. If the condition isn't true at that time, the ability is removed from the stack and does nothing. (…)

Now CR 603.6d says:

(…) Leaves-the-battlefield abilities (…) will trigger based on their existence, and the appearance of objects, prior to the event rather than afterward. The game has to “look back in time” to determine if these abilities trigger.

Does that mean that even with the current wording Lifeline's ability still triggers when all (i. e. two or more) creatures are destroyed, because when looking back in time, the ability sees all the creatures still on the battlefield? And it only can fail when it resolves and there are no creatures on the battlefield?

If so, you could use a Gift of Immortality and stack all triggered abilities in a way that Lifeline's ability fails for all opponent's creatures so that they remain in the graveyard, then Gift of Immortality brings one of your own creatures back, so that it can throw the Lifeline for all of your creatures, bringing them back at end of turn. Correct?

Mario

An intervening-if clause is checked at two moments:
- when the ability triggers,
- when the ability is about to resolve.

The triggered ability can then create a delayed trigger, but you check the intervening-if at the time the first ability resolves, not when the delayed trigger would resolve.

In Lifeline's case, if all creatures are destroyed at the same time:
- the first check is satisfied since you look just before the simultaneous zone change (since we're dealing with a leaves-the-battlefield trigger)
- the second check is done when Lifeline's triggered ability resolves, shortly after all creatures dies. Most likely the check will fail at that point (unless creatures have been reanimated) and the delayed trigger will not be created at all.

With Gift of Immortality, the Gift will bring its creature back, but Lifeline won't bring anything back.

Daniel Kitachewsky
L3, Paris, France
Rules NetRep

Oops, there's a mistake in my answer :).

I failed to realize that with Gift of Immortality the creature comes back right away and not at end of turn.

So in the full scenario, assuming you control the Lifeline, the creature enchanted with the Gift, and the Gift itself, if you stack the abilities in the following order:
1) Lifeline triggers for your creatures
2) Gift
3) Lifeline triggers for your opponent's creatures

then we resolve in the reverse order and get:
- Lifeline triggers for your opponent's creatures see no creatures on the battlefield and fail to create any delayed triggers,
- Gift brings back its creature,
- Lifeline triggers for your creatures all resolve and create delayed triggers, which will bring back your other creatures at end of turn.

Daniel Kitachewsky
L3, Paris, France
Rules NetRep