Myr Battlesphere [March of the Machine Commander]

When Myr Battlesphere enters the battlefield, create four 1/1 colorless Myr artifact creature tokens.

Whenever Myr Battlesphere attacks, you may tap X untapped Myr you control. If you do, Myr Battlesphere gets +X/+0 until end of turn and deals X damage to the player or planeswalker it's attacking.