What Does Echo Do?
Echo effectively gives "two for one" on a card. When you draw it from your deck, you immediately gain another copy of the same card in your hand. Does that second copy come from your deck? No. It just 'appears' from nowhere. So even if you only have one copy of (for example) Thunderstrike Dragon in your deck then when you draw it you get two copies in your hand.
Cards with Echo are thus a very efficient way of using space in your deck. The catch is that, as you'd expect, their power cost to play is a little higher than their stats would otherwise suggest.
Although you're limited to only four copies of a card in your deck, Echo provides to be a way of allowing you to have more than four copies in play during a game.
This page written for Eternal version 1.19, Card Set 1
Last updated: 17-04-2017