Cheapest priced copy (Theros): $2.49 at Card Kingdom, near mint, their price list as of .
Ashiok, Nightmare Weaver 
Legendary Planeswalker — Ashiok · Mythic · Theros · #188
+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.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $2.49 to $9.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros | 188 | Mythic | $2.49 | $9.99 |
5 rulings from Wizards of the Coast
- 15 September 2013If you put a creature card with bestow onto the battlefield, it will be a creature on the battlefield, not an Aura.
- 15 September 2013If you put a creature card with {X} in its mana cost onto the battlefield, the value of that X is 0.
- 15 September 2013Ashiok's second ability refers to any creature card exiled because of Ashiok's first or third ability.
- 15 September 2013If 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.
- 15 September 2013Ashiok'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.