Transmute

Transmute is the keyword ability of cards in Ravnica block aligned with the Dimir guild. It works by allowing a player to pay the transmute cost of a given card, discarding the card, then searching his or her library for a card with the same converted mana cost as the discarded card. It appears only on Black and Blue cards.

From the Comprehensive Rules:

• 502.48. Transmute
  • 502.48a Transmute is an activated ability that functions only while the card with transmute is in a player’s hand. “Transmute [cost]” means “[Cost], Discard this card: Search your library for a card with the same converted mana cost as the discarded card, reveal that card, and put it into your hand. Then shuffle your library. Play this ability only any time you could play a sorcery.”
  • 502.48b Although the transmute ability is playable only if the card is in a player’s hand, it continues to exist while the object is in play and in all other zones. Therefore objects with transmute will be affected by effects that depend on objects having one or more activated abilities.