Castle of Nothing

Castle of Nothing enters the battlefield tapped.
{t}: Add {b} to your mana pool.
{t}: Add {b}{b} to your mana pool. Spend this
mana only to cast Shade, Nightmare, or Horror
spells.
"The Nothing is a perilous place. It bends its form to
match the fears of its observer."
—Velin, Survivor of the Nothing.