The Eldest Reborn

When the first chapter ability of The Eldest Reborn resolves, the next opponent in turn order (or, if it's an opponent's turn, that opponent) chooses a creature or planeswalker they control, then each other opponent in turn order (if any) does the same. All chosen permanents are then sacrificed at the same time. Players will know the choices made by earlier players when making their choices.

As a Saga enters the battlefield, its controller puts a lore counter on it. As your precombat main phase begins (immediately after your draw step), you put another lore counter on each Saga you control. Putting a lore counter on a Saga in either of these ways doesn't use the stack.

Once the number of lore counters on a Saga is greater than or equal to the greatest number among its chapter abilities—in the Dominaria set, this is always three—the Saga's controller sacrifices it as soon as its chapter ability has left the stack, most likely by resolving or being countered. This state-based action doesn't use the stack.

In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, the permanent you control from The Eldest Reborn's final chapter ability is exiled.

If multiple chapter abilities trigger at the same time, their controller puts them on the stack in any order. If any of them require targets, those targets are chosen as you put the abilities on the stack, before any of those abilities resolve.

When the second chapter ability of The Eldest Reborn resolves, the next opponent in turn order (or, if it's an opponent's turn, that opponent) chooses a card in hand without revealing it, then each other opponent in turn order (if any) does the same. All chosen cards are then discarded at the same time.