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.
Lock one opponent out of casting each turn while plundering the top of their library, grinding incremental card advantage and tempo. You play a UB control shell with removal, stax, and theft effects, locking the table down and eventually winning with value engines or a finisher. Xanathar wants you to attack the social dynamics: shut down the biggest threat each upkeep.
Repeatable per-turn lockdown of an opponent's casting, neutralizing combo and control players
Free card advantage by playing off the top of a chosen opponent's library with any color mana
Lives in the best control colors for counters, removal, and card draw
Synergizes with other 'can't cast spells' and stax effects to broaden the lock
Only stops one opponent per upkeep, so a three-opponent table can still race or combo around you
Six mana and dies easily to removal, resetting your engine
Doesn't generate a fast clock on its own and games can drag
Vulnerable to activated abilities, mana abilities, and permanents already on board
Legendary Creature — Beholder
