What it does
I'm not aware of "Echo of the Lost" as an official Magic: The Gathering keyword or ability. It doesn't correspond to any printed keyword mechanic (like Echo, which is a real ability, or named cards I can verify).
It's possible you're thinking of Echo, the real mechanic: a creature with Echo requires you to pay its echo cost the first time it would untap during your upkeep (the turn after it comes under your control), or you sacrifice it. The common mistake is thinking you pay on cast—you actually pay later, and crucially, if it changes controllers (via blink or theft), the new controller pays echo again.
If "Echo of the Lost" is a specific card name, set name, or a custom/homebrew ability from Mythicwyrm, I'd need its actual text to explain it accurately. Could you share the card's wording?
Iconic cards with Echo of the Lost
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