Sejiri Merfolk is a uncommon Creature — Merfolk Soldier card from Worldwake (WWK), collector number 36. Current market price: $0.18 (foil $0.51), TCGplayer market prices via Scryfall.
As long as you control a Plains, this creature has first strike and lifelink. (Damage dealt by a creature with lifelink also causes its controller to gain that much life.)