Valentin, Dean of the Vein // Lisette, Dean of the Root

If a nontoken creature an opponent controls would die, and both Valentin and another effect are trying to exile it instead, that opponent chooses which effect to apply. If Valentin’s effect ends up not applying because of this choice, the triggered ability that allows you to pay {2} won’t trigger, even if the creature card ends up in exile.

Wotc — April 16, 2021

If you gain life at the same time a creature you control is dealt lethal damage, that creature dies before Lisette’s ability can put a +1/+1 counter on it.

Wotc — April 16, 2021

An ability that triggers “whenever you gain life” triggers just once for each life-gaining event, no matter how much life you gain.

Wotc — April 16, 2021

You may only pay {1} once for each life gain event.

Wotc — April 16, 2021

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, a “whenever you gain life” ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

Wotc — April 16, 2021

If you gain an amount of life “for each” of something, that life is gained as one event and the ability will trigger only once.

Wotc — April 16, 2021