Are you ready to dive into epic Commander battles
Such a huge and fundamental change to a mechanic is so rare that Companion continues to be one of the most controversial things added to Magic in the last few years, and regularly comes up as a point of discussion when talking about errata
Creature Artifact - Construct Terisian Mindbreaker Whenever this creature attacks, defending player mills half their library, rounded up
Honorable mentions: Top Card Draw Stapled to Creatures in White Where it lacks a bit in instants and sorceries, white makes up for it in creatures that draw
That's ideal for a few reasons, first, if a reanimation spell like Animate Dead was milled, you can now draw it after shuffling or Demonic Tutor
If you somehow can't discard a card (probably because you had no cards in hand and another effect prevented you from drawing cards), you won't put any counters on Cait no matter whether or not the defending player discarded a card or what that card's mana value is