Cavern of Souls

As Cavern of Souls enters the battlefield, choose
a creature type.
{t}: Add {c}
{t}: Add one mana of any color to your mana
pool. Use this mana only to cast creatures of
the chosen type. That spell can't be countered.
"The way is shut. It was made by those who are
dead, and the dead keep it. The way is shut."
-King of the Dead