Sentinel of the Nameless City is a rare Creature — Merfolk Warrior Scout card from The Lost Caverns of Ixalan (LCI), collector number 383. Current market price: $0.59 (foil $1.15), TCGplayer market prices via Scryfall.
Vigilance Whenever this creature enters or attacks, create a Map token. (It's an artifact with "{1}, {T}, Sacrifice this token: Target creature you control explores. Activate only as a sorcery.")