Xanathar, Guild Kingpin

The upkeep clause does two jobs at once, and the second is easy to miss because the first is so loud. Locking a single opponent out of casting spells for a full turn is a hard tax, but it is the mana-fixing rider that makes the theft real: playing off their library only matters if you can pay for whatever surfaces, and the license to spend mana as though it were any color removes the color-screw that usually neuters top-of-library plunder. That rider is the difference between an effect that reads well and one that actually cashes out, the failure mode of the long line of "play the top card of their library" cards that never quite pay for themselves. The design is a soft prison rather than a hard one: the chosen opponent still untaps, still attacks, still activates, still holds up their board. What they cannot do is respond on the stack, which turns your own turn into a window where nothing gets answered and every counterspell in their hand is dead paper. The 5/6 body keeps the engine honest as a real clock that survives combat, so the card is not a do-nothing centerpiece waiting to be removed. As a Beholder it leans into the flavor of the eye tyrant dominating a room by presence alone, but the mechanical read is cleaner than the fiction: a repeatable, targeted tax stapled to a resource-theft engine, with the color-agnostic mana doing the quiet heavy lifting.