Corpses of the Lost

Skeletons you control get +1/+0 and have haste.
When Corpses of the Lost enters, create a 2/2 black Skeleton Pirate creature token.
At the beginning of your end step, if you descended this turn, you may pay 1 life. If you do, return Corpses of the Lost to its owner's hand. (You descended if a permanent card was put into your graveyard from anywhere.)

The Lost Caverns of Ixalan

Skeletons you control get +1/+0 and have haste.
When Corpses of the Lost enters the battlefield, create a 2/2 black Skeleton Pirate creature token.
At the beginning of your end step, if you descended this turn, you may pay 1 life. If you do, return Corpses of the Lost to its owner's hand. (You descended if a permanent card was put into your graveyard from anywhere.)

Skeletons you control get +1/+0 and have haste.
When Corpses of the Lost enters the battlefield, create a 2/2 black Skeleton Pirate creature token.
At the beginning of your end step, if you descended this turn, you may pay 1 life. If you do, return Corpses of the Lost to its owner's hand.

The Lost Caverns of Ixalan Promos

A permanent card is an artifact, battle, creature, enchantment, land, or planeswalker card. Tokens are not cards, and while tokens are put into the graveyard before ceasing to exist, that action doesn't count as a player having descended.

Abilities that begin with "At the beginning of your end step, if you descended this turn" will trigger only once during your end step, no matter how many times you descended this turn. However, if you haven't descended this turn as your end step begins, the ability won't trigger at all. It's not possible to put a permanent card into your graveyard during the end step in time to have the ability trigger.

In either case, it doesn't matter if those cards are still in that player's graveyard.

Multiple cards have abilities that begin with "At the beginning of your end step, if you descended this turn." These cards don't need to have been under your control at the time you descended. For example, if a permanent card is put into your graveyard during your first main phase and you cast Stalactite Stalker your second main phase, its ability will trigger at the beginning of your end step.

Some cards refer to a player who has "descended this turn." This means that a permanent card has been put into that player's graveyard from anywhere this turn.

Some cards refer to the number of times a player descended this turn. Those cards care about the number of permanent cards put into that player's graveyard from anywhere this turn.

You decide to pay 1 life as the last ability is resolving. If you decide to pay 1 life and Corpses of the Lost is still on the battlefield, it will immediately return to its owner's hand. No player may take any actions in between you deciding to pay life and it returning this way.