Myr Battlesphere (2XM) from Double Masters, available at 401 Games Canada.

• Rarity: Rare
• Finish: Normal
• Artist: Franz Vohwinkel
• Card Text:
  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.