Several cards have an eternalize cost that includes "Discard a card." You can't discard the card with eternalize to pay its own cost because the card has to be in your graveyard to begin activating its eternalize ability.

For each card with eternalize, a corresponding game play supplement token can be found in some Hour of Devastation booster packs. These supplements are not required to play with cards with eternalize; you can use the same items to represent an eternalized token as you would any other token.

Once you've activated an eternalize ability, the card is immediately exiled. Opponents can't try to stop the ability by exiling the card with an effect such as that of Crook of Condemnation.

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.

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, such as with Crook of Condemnation, if it's legal for you to do so.

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.