Thought Erasure

• 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.

• WWizards of the Coast2018-10-05
  Official Ruling
  You surveil 1 even if the opponent doesn't discard a card, perhaps because they had no cards in hand at all.
• WWizards of the Coast2024-01-12
  Official Ruling
  Some spells that instruct you to surveil require targets. You can't cast a spell without choosing legal targets. If all of those targets become illegal, the spell doesn't resolve and you won't surveil.
• 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