Tiamat is a mythic Legendary Creature — Dragon God card from The List (PLST), collector number AFR-235. Current market price: $23.78, TCGplayer market prices via Scryfall.
Flying When Tiamat enters, if you cast it, search your library for up to five Dragon cards not named Tiamat that each have different names, reveal them, put them into your hand, then shuffle.