Ashiok, Muse des Albtraums

Any effect that exiles a card exiles it face up unless that effect specifies otherwise.

If Ashiok's second ability returns a token to its owner's hand, that player can't exile that token from their hand. They must exile a card from their hand if possible.

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, such as that of Thrill of Possibility, those must be paid to cast the card.

If you wish to cast any exiled cards, you do so as part of the resolution of Ashiok's last ability. You can't wait to cast them later. Timing permissions based on a card's type are ignored.

In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any spells and/or permanents you control from Ashiok's last ability are exiled.

The card exiled while resolving Ashiok's second ability may be the one that was returned to its owner's hand.

You may cast any face-up cards your opponents own from exile while resolving Ashiok's last ability, not just those exiled by Ashiok's second ability or by Ashiok's Nightmare tokens.