Timeless Witness

• WOTC Official Reminder Textofficial rules
  Eternalize — Eternalize [cost] ([cost], Exile this card from your graveyard: Create a token that's a copy of it, except it's 4/4, black, a Zombie, and has no mana cost. Eternalize only as a sorcery.)

• WWizards of the Coast2021-06-18
  Official Ruling
  The token is a Zombie in addition to its other types and is black instead of its other colors. Its base power and toughness are 4/4. It has no mana cost, and thus its mana value is 0. These are copiable values of the token that other effects may copy.
• WWizards of the Coast2021-06-18
  Official Ruling
  If a creature card with eternalize is put into your graveyard during your main phase, you'll have priority immediately afterward. You can activate its eternalize ability before any player can try to exile it.
• WWizards of the Coast2021-06-18
  Official Ruling
  Once you've activated an eternalize ability, the card is immediately exiled. Opponents can't try to stop the ability by exiling the card.
• WWizards of the Coast2021-06-18
  Official Ruling
  The token copies exactly what was printed on the original card and nothing else, except the characteristics specifically modified by eternalize. It doesn't copy any information about the object the card was before it was put into your graveyard.

Rulings via Scryfall