Echo

Echo is an ability whereby the card can be played multiple times in one turn. This is accomplished through copying the card to the user's hand until the end of their turn. It was informally introduced in Kobolds & Catacombs as Unstable Evolution, and officially debuted in The Witchwood.

Echo copies have a blue glow and black smoke around them.

Cards with Echo
This section lists cards which have the ability to Echo when played.

Echo-generating cards
This section lists cards which generate cards that have the ability to Echo.

Echo-granting cards
This section lists cards which grant the Echo ability to other cards or to itself.

Cards with the Echo mechanic
When Echo was announced, Blizzard confirmed they would not be modifying already-existing cards to have the updated keyword because the mechanics are not completely identical to Echo. The following cards have the mechanic without the keyword.

History
Echo was inspired by "Ghostly", an earlier mechanic experimented with during the development of The Witchwood, when the expansion was instead called Murder on the Gilnean Express. Cards with Ghostly discarded themselves at the end of the player's turn. Cards that only had Ghostly as their card text essentially required the player to play the card the turn it was drawn, while other cards added Ghostly copies of themselves to the player's hand. As Blizzard kept iterating, they found that the most fun version of Ghostly was the cards that replicated themselves, as they could both be useful during the early game and could be powerful during the late game if the player sunk extra mana into them. Furthermore, deciding how long to hold on to them to eke out more value proved to be skill-testing and interesting. As the setting of the expansion evolved from a train to a haunted forest, Ghostly "became more and more the perfect flavor for the set", eventually becoming Echo.

Trivia

 * Although Blizzard did not add Echo to Unstable Evolution or Witch's Brew, it was given to SN1P-SN4P. This is likely because SN1P-SN4P rotated out of Standard alongside the other Echo cards, meaning there won't be any confusion.
 * Prior to Echo's nerf, it was possible to play infinite copies of an Echo card in a single turn if there was a cost reduction effect for both the original card and its subsequent copies, such as Echo spells being reduced to 0 mana by Millhouse Manastorm. SN1P-SN4P was a notable offender for this condition as its Magnetic meant that it was possible to play infinite SN1P-SN4Ps without running out of board space. This was the reason that Reckless Experimenter was nerfed to be unable to reduce Deathrattle minions below 1 (as SN1P-SN4P also has Deathrattle), but SN1P-SN4P Warlock managed to do so anyway by using Mechwarper and Summoning Portal to play as many SN1P-SN4Ps as possible in a single turn, an exploit so prominent that it was the reason that Echo was nerfed in the first place. With its limitation already applied to all Echo cards, Reckless Experimenter's nerf was reverted as well.
 * "Repeatable this turn" cards are unaffected by the Echo nerf so they can still be played indefinitely, but the ability is exclusive to Shaman cards. However, this interaction can still be seen upon somehow gaining a Sorcerer's Apprentice or Radiant Elemental.

Patch changes

 * Copies of cards with the Echo keyword can no longer cost less than (1).
 * Added.