Ashiok, Nightmare Weaver

[+2]: Exile the top three cards of target opponent's library.
[−X]: Put a creature card with mana value X exiled with Ashiok onto the battlefield under your control. That creature is a Nightmare in addition to its other types.
[−10]: Exile all cards from all opponents' hands and graveyards.

Pioneer Masters

Theros

+2: Exile the top three cards of target opponent's library.
-X: Put a creature card with converted mana cost X exiled with Ashiok, Nightmare Weaver onto the battlefield under your control. That creature is a Nightmare in addition to its other types.
-10: Exile all cards from all opponents' hands and graveyards.

Ashiok's second ability doesn't target any creature card. You choose which creature card to return when that ability resolves, but you must choose one with mana value equal to the value you chose for X when activating the ability.

Ashiok's second ability refers to any creature card exiled because of Ashiok's first or third ability.

If Ashiok leaves the battlefield, and later another Ashiok enters the battlefield, it is a new object (even if the two were represented by the same card). Creature cards exiled by the original Ashiok can't be put onto the battlefield by the second ability of the new Ashiok.

If you put a creature card with bestow onto the battlefield, it will be a creature on the battlefield, not an Aura.

If you put a creature card with {X} in its mana cost onto the battlefield, the value of that X is 0.