Spells that force your opponent to mill half their library are common for mill decks, but for it to be an additional cost isn't quite so common
Weve had a tough couple of years in regards to bannings, but 2021 seemed pretty stable as a whole
Its hard to recover if your opponent plays two of these
-sigh- Commanders emphasis on color identity in deckbuilding has successfully kept Krrik, Son of Yawgmoth from being a huge problem in the format as a whole, but its still a powerhouse that should be removed or countered on sight against decks with a large or exclusive emphasis on black, as 4 mana can easily become as much as 16, more if life gain enters the equation
It is hard not to play a Library, the card is so lovely
But thats enough of an engine to total change the deck (Im even running Petrified Field to grab a dead Bazaar in case someone gets into a Destroy on Sight state of mind)