Thundering Mightmare (VOC) from Crimson Vow Commander, available at 401 Games Canada.

• Rarity: Rare
• Finish: Normal
• Artist: Lorenzo Mastroianni
• Card Text:
  Soulbond (You may pair this creature with another unpaired creature when either enters the battlefield. They remain paired for as long as you control both of them.)
  As long as Thundering Mightmare is paired with another creature, each of those creatures has "Whenever an opponent casts a spell, put a +1/+1 counter on this creature."