“Clear Temple Ancestral Tomb” is the Misc quest in Skyrim - Dragonborn
Head into the Temple found in Raven Rock. Talk with Elder Othreloth after he has talked with Adril Arano. He will mention that Ash Spawn have infested part of the temple and he needs someone to clear them out. Talk with him after that conversation and agree to clear out the Tomb.
Go outside and travel over to the Morvayn Ancestral Tomb. Go in to the Tomb and make your way to the Ancestral Tomb found in the back of that tomb. This can also be accessed from the Ulen Ancestral Tomb and the Raven Rock Temple.
Inside the tomb you will encounter a number of leveled Ash Spawn. Take your time and keep your distance since they are largely melee fighters. Defeating most of them will cause their leader, an Ash Spawn Immolator to appear.
When the tomb is cleared, head back to the Temple and talk with Elder Othreloth again to collect your reward.