Cavern of Souls
(LSV) This is certainly a powerful card, and I expect the implications of printing it to last a long time, especially since it has applications in Eternal formats.
Color: LandCard Text: As Cavern of Souls enters the battlefield, choose a creature type.T: Add 1 to your mana pool.T: Add one mana of any color to your mana pool. Spend this mana only to cast creature spells of the chosen creature type. If this mana is spent on a spell, that spell can't be countered.Rarity: RCard Type: LandArtist: Cliff ChildsName: Cavern of SoulsFinish: RegularCard Number: 226Set Name: Avacyn Restored