Spawning Pool is a uncommon Land card from Urza's Legacy (ULG), collector number 142. Current market price: $0.36 (foil $12.87), TCGplayer market prices via Scryfall.
This land enters tapped. {T}: Add {B}. {1}{B}: This land becomes a 1/1 black Skeleton creature with "{B}: Regenerate this creature" until end of turn. It's still a land. (If it regenerates, the next time it would be destroyed this turn, instead tap it, remove it from combat, and heal all damage on it.)