Mephitic Vapors

• 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
  Mephitic Vapors affects only creatures on the battlefield at the time it resolves. Creatures that enter the battlefield later in the turn won't get -1/-1.
• WWizards of the Coast2018-10-05
  Official Ruling
  Creatures that are going to die after getting -1/-1 will still be on the battlefield while you surveil. They won't be put into their owners' graveyards until after Mephitic Vapors is entirely finished resolving. If any of their abilities trigger on surveilling, those abilities will trigger but won't resolve until after the creatures have left the battlefield.
• 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