Prison Realm

• WOTC Official Reminder Textofficial rules
  Scry — Look at the top N cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.

• WWizards of the Coast2019-05-03
  Official Ruling
  You’ll still scry 1 even if Prison Realm’s first ability has no legal target or if that target becomes an illegal target before the ability resolves.
• WWizards of the Coast2019-05-03
  Official Ruling
  If a token is exiled this way, it will cease to exist and won’t return to the battlefield.
• WWizards of the Coast2019-05-03
  Official Ruling
  If Prison Realm leaves the battlefield before its first triggered ability resolves, the target permanent won’t be exiled.
• WWizards of the Coast2019-05-03
  Official Ruling
  Auras attached to the exiled permanent will be put into their owners’ graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled permanent will cease to exist. When the card returns to the battlefield, it will be a new object with no connection to the card that was exiled.

Rulings via Scryfall