Cheapest priced copy (Adventures in the Forgotten Realms): $3.49 at Card Kingdom, near mint, their price list as of .
Xanathar, Guild Kingpin 
Legendary Creature — Beholder · 5/6 · Mythic · Adventures in the Forgotten Realms · #239
At the beginning of your upkeep, choose target opponent. Until end of turn, that player can't cast spells, you may look at the top card of their library any time, you may play the top card of their library, and you may spend mana as though it were mana of any color to cast spells this way.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $2.99 to $16.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Adventures in the Forgotten Realms | 239 | Mythic | $3.49 | $2.99 |
| Adventures in the Forgotten Realms | 391 | Mythic | $3.49 | $3.99 |
| Adventures in the Forgotten Realms Promos | 239a | Mythic | — | $16.99 |
| Adventures in the Forgotten Realms Promos | 239p | Mythic | $7.49 | $5.99 |
| Adventures in the Forgotten Realms Promos | 239s | Mythic | — | $6.99 |
| Secret Lair Drop | 1785 | Mythic | $5.49Out of stock | $6.99 |
5 rulings from Wizards of the Coast
- 23 July 2021The target opponent may still cast spells in response to Xanathar, Guild Kingpin's ability in your upkeep. If that player has somehow become an illegal target for the ability as it tries to resolve, the ability does nothing. They may cast spells and you cannot look at or play the top card of their library.
- 23 July 2021You'll still pay all costs for spells cast this way, including additional costs. You may also pay alternative costs if any are available.
- 23 July 2021During your turn, you can look at the top card of the chosen opponent's library whenever you want (with one restriction; see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.
- 23 July 2021If the top card of that library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast a spell from the top of that library, you can't look at the next card until you're done paying for that spell.
- 23 July 2021Xanathar doesn't change when you can cast spells or play lands. You must still follow all normal timing rules and land play limits.