Enduring Tenacity is a rare Enchantment Creature — Snake Glimmer card from Duskmourn: House of Horror Promos (PDSK), collector number 95p. Current market price: $13.16 (foil $13.28), TCGplayer market prices via Scryfall.
Whenever you gain life, target opponent loses that much life. When Enduring Tenacity dies, if it was a creature, return it to the battlefield under its owner's control. It's an enchantment. (It's not a creature.)