Thief of Sanity

Mana cost: {1}{U}{B}
Type: Creature — Specter
Power / toughness: 2 / 2
Set: Guilds of Ravnica #205 (rare)
Price: $0.77 USD

Flying
Whenever this creature deals combat damage to a player, look at the top three cards of that player's library, exile one of them face down, then put the rest into their graveyard. You may cast that card for as long as it remains exiled, and mana of any type can be spent to cast that spell.

Thief of Sanity has 7 printings, first in Guilds of Ravnica (2018). EDHREC ranks it #3,195 among Commander cards. It appears most in Gonti, Canny Acquisitor decks (9,543 on EDHREC). 3 public decks on TCG run it. It has 5 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: legal
standard: not legal
brawl: legal

Public decks that run Thief of Sanity

Grand Larceny - Outlaws of Thunder Junction Commander, led by Gonti, Canny Acquisitor
Obscura Operation - New Capenna Commander, led by Kamiz, Obscura Oculus
Odds and Ends - Secret Lair Commander, led by Yennett, Cryptic Sovereign

Commanders that run Thief of Sanity most

Deck counts from EDHREC.

Gonti, Canny Acquisitor: 9,543 decks
Tasha, the Witch Queen: 4,671 decks
Don Andres, the Renegade: 4,497 decks
Jin Sakai, Ghost of Tsushima: 2,251 decks
Nathan Drake, Treasure Hunter: 2,155 decks
Felix Five-Boots: 2,106 decks
Xanathar, Guild Kingpin: 1,961 decks
Kamiz, Obscura Oculus: 1,645 decks

Often played with Thief of Sanity

Cards that show up in the same decks on EDHREC.

Rulings

2018-10-05: Casting an exiled card causes it to leave exile. You can't cast it multiple times.
2018-10-05: You can still cast the exiled card if Thief of Sanity leaves the battlefield or leaves your control. If another player gains control of Thief of Sanity, that player can't cast the exiled card.
2018-10-05: An effect that instructs you to "cast" a card doesn't allow you to play lands.
2018-10-05: Thief of Sanity's effect doesn't change when you can cast the exiled card. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty.
2018-10-05: In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any spells or permanents you control from Thief of Sanity's effect are exiled, and any of the face-down exiled cards remain face down indefinitely. No player may look at them.