Aven Mindcensor

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2021-03-19
  Official Ruling
  For effects that check whether a player searched a library or may search a library, searching the top four cards of that library counts as searching that library.
• WWizards of the Coast2021-03-19
  Official Ruling
  After the search is complete, the entire library is shuffled.
• WWizards of the Coast2021-03-19
  Official Ruling
  Each search works exactly the same as normal except that the opponent doing the searching sees only the top four cards of that library. They can't look at the other cards in that library at all. Cards not in the top four cards of the library can't be found in the search, even if they're identifiable in some manner.
• WWizards of the Coast2021-03-19
  Official Ruling
  If an effect uses the word "search" once but allows an opponent to search for multiple cards, the top four cards remain the same throughout the entire search. Finding one card won't cause the fifth card to become included in the search.

Rulings via Scryfall