Sheoldred

You can activate Sheoldred’s last ability if any opponent has eight or more cards in their graveyard, not necessarily all of them.

Once you’ve activated Sheoldred’s last ability, it doesn’t matter what happens to the number of cards in opponents’ graveyards. The ability will resolve even if no opponent has eight or more cards in their graveyard by that time.

To resolve Sheoldred’s enters-the-battlefield ability in a multiplayer game, the next opponent in turn order (or, if it’s an opponent’s turn, the opponent whose turn it is) chooses a creature they control to sacrifice. Then each other opponent in turn order does the same. Then those creatures are sacrificed at the same time.