Myr Battlesphere is a rare Artifact Creature — Myr Construct card from Modern Horizons 3 Commander (M3C), collector number 301. Current market price: $0.37, 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.