Swimmer in Nightmares is a uncommon Creature — Nightmare Merfolk card from Theros Beyond Death (THB), collector number 275. Current market price: $0.22, TCGplayer market prices via Scryfall.
This creature gets +3/+0 as long as there are ten or more cards in a single graveyard. This creature can't be blocked as long as you control an Ashiok planeswalker.