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.

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.

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.