Elemental Mastery is a rare Enchantment — Aura card from Shadowmoor (SHM), collector number 90. Current market price: $4.31 (foil $18.46), TCGplayer market prices via Scryfall.
Enchant creature Enchanted creature has "{T}: Create X 1/1 red Elemental creature tokens with haste, where X is this creature's power. Exile them at the beginning of the next end step."