Myr Battlesphere is a rare Artifact Creature — Myr Construct card from Double Masters (2XM), collector number 276. Current market price: $0.44 (foil $2.81), TCGplayer market prices via Scryfall.
When this creature enters, create four 1/1 colorless Myr artifact creature tokens. Whenever this creature attacks, you may tap X untapped Myr you control. If you do, this creature gets +X/+0 until end of turn and deals X damage to the player or planeswalker it's attacking.