Demon Bolt — ruling 1

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.

Demon Bolt — ruling 2

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.

Demon Bolt — ruling 3

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.

Demon Bolt : règles, fonctionnement et interactions (MTG)

Demon Bolt is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Demon Bolt deals 4 damage to target creature or planeswalker. Foretell (During your turn, you may pay and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

How it works : Demon Bolt

Original English text — Wizards of the Coast / Scryfall

#1February 5, 2021· Wizards of the Coast
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.
Card named in this ruling
Foretell
#2February 5, 2021· Wizards of the Coast
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.
Card named in this ruling
Foretell
#3February 5, 2021· Wizards of the Coast
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.
Card named in this ruling
Foretell