Tiamat

The Dragon Queen
The Nine Hells is ruled by the Overlord Asmodeus, but not all who dwell there bow to his will, one who was worshiped as a goddess before Asmodeus's rebellion against He Who Was. The great Tiamat, the queen of chromatic dragon kind. She plots in her dragonspawn pits on how to turn the mortal world to darkness, claim the vast unending treasure of the undeserving, and slay her rival and brother the dragon god Bahamut.

How she came to be a resident of Avernus and what her agreement with Asmodeus is no one but they know. Whatever it may be, the Dragon Queen remains separate from devilish ambition, and any devil that attempt to force her into a scheme she hasn't agreed to will find that the goddess is just as dangerous as their Overlord. Anyone that can make the proud dragons bow, can make anyone bow.