Tergrid's Shadow

If you're casting a foretold card from exile for its foretell cost, you can't choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.

Casting a foretold card from exile follows the timing rules for that card. If you foretell an instant card, you can cast it as soon as the next player's turn. In most cases, if you foretell a card that isn't an instant (or doesn't have flash), you'll have to wait until your next turn to cast it.

As Tergrid's Shadow resolves, first the player whose turn it is chooses two creatures they control, then each other player in turn order does the same, knowing the choices made before them. Then all the chosen creatures are sacrificed at the same time.