Dream Devourer

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.

The last ability triggers when you exile a card from your hand using foretell, not when you cast a foretold card from exile.

It doesn’t matter if Dream Devourer is still on the battlefield or still under your control as you cast the spell from exile.

Cards that are foretold by virtue of Dream Devourer’s ability won’t continue to have the foretell ability once they leave your hand, but you can still cast them from exile on a later turn for the foretell costs they were given by Dream Devourer.

The reduction of the foretell cost applies only to generic mana in the foretell cost. It can’t reduce requirements of specific colors of mana. For example, if you foretell a card in your hand with mana cost {1}{G}{G}, its foretell cost will be {G}{G}.

Because exiling a card with foretell from your hand is a special action, you can do so any time you have priority during your turn, including in response to spells and abilities. Once you announce you’re taking the action, no other player can respond by trying to remove the card from your hand.