Creeping Tar Pit (WWK) from Worldwake, available at 401 Games Canada.

• Rarity: Rare
• Finish: Normal
• Artist: Jason Felix
• Card Text:
  Creeping Tar Pit enters the battlefield tapped.
  {T}: Add U or B to your mana pool.
  1UB: Until end of turn, Creeping Tar Pit becomes a 3/2 blue and black Elemental creature and is unblockable. It's still a land.