Mox Granite

Hellbent - {t}: Add {r} or {b}. Only activate this
ability if you have no cards in hand.
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"The Rakdos need lots of supply to keep their circus of
freaks going, so they made this, a rock that provides
mana for those who know how to use it. It is one of
the only civilized inventions of the Rakdos in my
ideal."
- Dovin Baan