Xanathar, Guild Kingpin — ruling 1

Xanathar doesn't change when you can cast spells or play lands. You must still follow all normal timing rules and land play limits.

Xanathar, Guild Kingpin — ruling 2

The 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.

Xanathar, Guild Kingpin — ruling 3

During 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.

Xanathar, Guild Kingpin — ruling 4

If 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.

Xanathar, Guild Kingpin — ruling 5

You'll still pay all costs for spells cast this way, including additional costs. You may also pay alternative costs if any are available.

Xanathar, Guild Kingpin : règles, fonctionnement et interactions (MTG)

Xanathar, Guild Kingpin 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

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.

How it works : Xanathar, Guild Kingpin

Original English text — Wizards of the Coast / Scryfall

#1July 23, 2021· Wizards of the Coast
Xanathar doesn't change when you can cast spells or play lands. You must still follow all normal timing rules and land play limits.
No other card is named in this ruling.
#2July 23, 2021· Wizards of the Coast
The 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.
No other card is named in this ruling.
#3July 23, 2021· Wizards of the Coast
During 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.
No other card is named in this ruling.
#4July 23, 2021· Wizards of the Coast
If 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.
No other card is named in this ruling.
#5July 23, 2021· Wizards of the Coast
You'll still pay all costs for spells cast this way, including additional costs. You may also pay alternative costs if any are available.
No other card is named in this ruling.