2020-01-24 : If Underworld Sentinel leaves the battlefield without dying, the exiled creature cards are exiled for the rest of the game. If Underworld Sentinel returns to the battlefield later, it’ll be a new object with no memory of the creatures it exiled previously.

2020-01-24 : If you gain control of another player’s Underworld Sentinel and it then dies, you’ll put the exiled cards onto the battlefield under your control.

2020-01-24 : In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any permanents you control from Underworld Sentinel but don’t own are exiled.

2020-01-24 : Underworld Sentinel can attack even if there’s no creature card in your graveyard to exile.