Nightmare Shepherd is a rare Enchantment Creature — Demon card from Theros Beyond Death Promos (PTHB), collector number 108p. Current market price: $0.48 (foil $0.50), TCGplayer market prices via Scryfall.
Flying Whenever another nontoken creature you control dies, you may exile it. If you do, create a token that's a copy of that creature, except it's 1/1 and it's a Nightmare in addition to its other types.