Guilds of Ravnica

Narcomoeba | Creature | Illusion | Flying When Narcomoeba is put into your graveyard from your library, you may put it onto the battlefield. | Howard Lyon

Omnispell Adept | Creature | Human Wizard | {2}{U}, {T}: You may cast an instant or sorcery spell from your hand without paying its mana cost. | Jason Rainville

Etrata, the Silencer | Legendary Creature | Vampire Assassin | Etrata, the Silencer can't be blocked. Whenever Etrata deals combat damage to a player, exile target creature that player controls and put a hit counter on that card. That player loses the game if they own three or more exiled cards with hit counters on them. Etrata's owner shuffles Etrata into their library. | Bastien L. Deharme

Thief of Sanity | Creature | Specter | Flying Whenever Thief of Sanity 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 look at and cast that card for as long as it remains exiled, and you may spend mana as though it were mana of any type to cast that spell. | Igor Kieryluk

Niv-Mizzet, Parun | Legendary Creature | Dragon Wizard | This spell can't be countered. Flying Whenever you draw a card, Niv-Mizzet, Parun deals 1 damage to any target. Whenever a player casts an instant or sorcery spell, you draw a card. | Svetlin Velinov

Chamber Sentry | Artifact Creature | Construct | Chamber Sentry enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it. {X}, {T}, Remove X +1/+1 counters from Chamber Sentry: It deals X damage to any target. {W}{U}{B}{R}{G}: Return Chamber Sentry from your graveyard to your hand. | Adam Paquette