Underworld Sentinel

• WWizards of the Coast2020-01-24
  Official Ruling
  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.
• WWizards of the Coast2020-01-24
  Official Ruling
  Underworld Sentinel can attack even if there’s no creature card in your graveyard to exile.
• WWizards of the Coast2020-01-24
  Official Ruling
  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.
• WWizards of the Coast2020-01-24
  Official Ruling
  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.

Rulings via Scryfall