Othreloth asks the Dragonborn to help in removing the Ash Spawn that have appeared in the tombs below the temple. When entering the tomb, nine to eleven Ash Spawn will attack. After clearing it out and heading back to Othreloth, the Dragonborn will receive a sum of gold, which is dependent on level.
What are the Ancestral Tombs?: "They are where we inter our dead. They are sacred places, meant to honor their passing. Sadly, we've been unable to use the Ancestral Tomb beneath the Temple as of late, and have resorted to other locations to scatter the remains."
Why can't you use the tomb?: "Those foul vermin... the abominations we call ash spawn have risen from the ashes of our own ancestors. They're befouling the memories of our forefathers by defiling their remains."
I'll cleanse the tomb for you: "Thank you, wanderer. That would be most kind. Here, this key should open the door to the tomb."
The Temple's tomb has been cleansed: "Spirits be praised! I'm hoping you didn't suffer much at the hands of those foul beings. Please, accept this as a token of my thanks."