What is Echo in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Echo keyword.
Repeatable the turn you play this.
Echo is a keyword that essentially means that you can keep playing the same card multiple times in a turn provided you have the mana to spend. After you play an Echo card, it will add itself back to your hand with an aura around it meaning that it will vanish at the end of the turn.
The mechanic was unofficially debuted in Kobolds & Catacombs with Unstable Evolution. It was made an official keyword in the next set, The Witchwood, although Unstable Evolution was not changed to have the keyword. As a keyworded mechanic, Echo is exclusive to The Witchwood with the exception of SN1P-SN4P which was retroactively added to The Boomsday Project, the very next set.
As of Patch 22.214.171.124060, it is no longer possible for Echo cards to cost less than 1 mana. As Unstable Evolution and Witch's Brew do not actually have the Echo keyword, they are not affected by this patch and are allowed to cost 0 through means like Sorcerer's Apprentice. Due to the lack of any Echo synergy, a card having Echo instead of "Repeatable this turn" can strangely be viewed as a nerf.
Mistwraith is the only card in the entire game which has any synergy with the Echo mechanic.
Keywords that Work Great with Echo
Due to the nature of the keyword, practically every keyword can work great with Echo. Many Echo cards already have some bonus keywords on them, but any minion can be granted Echo through Glinda Crowskin.
Playing Against Echo in Hearthstone
Since most Echo minions are small, they are fairly susceptible to board clears if many of them are played in one turn.
If your opponent plays a Glinda Crowskin, it may be in your best interest to kill it as soon as possible, or else it can snowball the game heavily.
Example Hearthstone Echo Cards
Phantom Militia is a basic form of Echo, being able to play several Taunt minions in one turn. This can be useful with Fire Plume's Heart, as one card can potentially complete 43% of the Quest by itself.
Sound the Bells! allows you to keep giving Divine Strengths to minions. Use it all on one minion to buff it to hugely buff it, or spread it out across your field and let everything share the boosted stats. Particularly hilarious if you manage to get it cost 0, although this is no longer possible.
SN1P-SN4P is the only Echo card not from The Witchwood. It's a very strong card, essentially a stronger Harvest Golem that replicates itself. It uses Magnetic, and as a Mech, it can always attach to itself again. Each instance of the Deathrattle will also stack for each time it is Magnetized to each other.