Molag Bal's Quest

While in a Shrine to Molag Bal, the Daedric Prince spoke to me at the altar. He was angered, and I was afraid. His anger, however, was not directed at me, but rather toward one of his minions. Menta Na, a Daedroth Monarch, has grown lazy, and no longer does Molag Bal's bidding. If I kill Menta Na in his dungeon Kora-Dur, returning his soul to the Outer Realms where Molag Bal can punish him, then Molag Bal will reward me well.

I've killed the Daedroth Menta Na.

I've returned to the Shrine of Molag Bal, having killed the Daedroth Monarch. Molag Bal was true to his word, and has rewarded me with his mace, the Mace of Molag Bal.