Note: This only becomes available once you’ve completed both of the following side-quests 'Healing a House' and 'Reluctant Steward' .
Travel to Tel Mithryn and you’ll encounter several Ash Spawn outside. Upon killing them, head into the main tower and notify Neloth of the attack. He states that its not a coincidence that the events that have taken place occurred, and sbelieves that someone must be behind the attacks.
Find the Source
He gives you his ring of tracking which will help identify the culprit. Put on the ring and head east along the coast of Tel Mithryn to a small graveyard. When within a hundred feet of the location, a sarcophagus starts to glow purple, and fade.
This is Ildari Sarothril’s sarcophagus and inside is a Heart Stone. Collect it, then return to Neloth to tell him the stone was in Ildari’s grave. Neloth thinks Ildari, his former apprentice, is behind the attacks. He believed she was dead, but there was no body in the grave.
Traveling to Highpoint Tower, the entry was previously blocked, but the ash has been cleared. The path to the door can be reached by a nondescript crevice on the northern side of the mound. Ash spawn are guarding the entrance which will need to be taken care of. Upon entering Highpoint Tower, head down the stairs.
Deep underground, the fort contains a prison where Niyya, a miner, is locked up. If released, she reveals that she and the other miners took pity on Ildari, thinking she was badly injured, and she repaid their kindness by attacking them in the night. She imprisoned most of them for her personal experiments.
Further exploration will reveal a large cavernous area where Ildari is holed up with Ash Guardians and ash spawn. She can be found and fought in a final chamber, and the battle ends with the Dragonborn ripping out the Heart Stone that was keeping her alive. Now return back to Neloth and claim your reward!