Dream Eater

• WOTC Official Reminder Textofficial rules
  Surveil — Look at the top N cards of your library. Put any number of them into your graveyard and the rest on top of your library in any order.
• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2018-10-05
  Official Ruling
  Dream Eater's triggered ability goes on the stack without a target. While that ability is resolving, after you've surveilled, the reflexive triggered ability triggers and you pick a target nonland permanent to be returned to its owner's hand. This is different from effects that say "If you do . . ." in that you choose the target after having surveilled.
• WWizards of the Coast2018-10-05
  Official Ruling
  Dream Eater's reflexive triggered ability triggers even if you have fewer than four cards in your library to surveil.
• WWizards of the Coast2024-01-12
  Official Ruling
  When you surveil, you may put all the cards you look at back on top of your library, you may put all of those cards into your graveyard, or you may put some of those cards on top and the rest of them into your graveyard.
• WWizards of the Coast2024-01-12
  Official Ruling
  You perform the actions stated on a card in sequence. For some spells and abilities, you'll surveil last. For others, you'll surveil and then perform other actions.

Rulings via Scryfall