Deepfathom Echo is a rare Creature — Merfolk Spirit card from The Lost Caverns of Ixalan (LCI), collector number 385. Current market price: $0.35 (foil $0.42), TCGplayer market prices via Scryfall.
At the beginning of combat on your turn, this creature explores. Then you may have it become a copy of another creature you control until end of turn. (To have this creature explore, reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)