Dungeon Master's Guide, *5th Edition (2014)*

Acererak the Archlich

When Acererak the Archlich enters, if you haven't completed Tomb of Annihilation, return Acererak the Archlich to its owner's hand and venture into the dungeon.
Whenever Acererak the Archlich attacks, for each opponent, you create a 2/2 black Zombie creature token unless that player sacrifices a creature of their choice.

Secret Lair Drop

Magic Online Promos

Adventures in the Forgotten Realms

Adventures in the Forgotten Realms Promos

Acererak the Archlich has an intervening if clause in its first triggered ability. If you have completed Tomb of Annihilation, the ability won't trigger at all. If it does trigger, but you somehow complete Tomb of Annihilation while the ability is on the stack, then the ability won't do anything. You won't return Acererak to its owner's hand and you won't venture into the dungeon.

If Acererak is no longer on the battlefield as its first ability resolves and you haven't completed Tomb of Annihilation, you will still venture into the dungeon.

In a multiplayer game, each opponent decides in turn order whether or not they will sacrifice a creature and which one they will sacrifice. Each opponent knows the choices made before they make their own choice. Then, once all opponents have chosen, creatures are sacrificed and tokens are created simultaneously.