Old Friends

Background
Ash Spawn have been attacking Tel Mithryn, one of their attacks killing Neloth's previous steward, and another causing a part of Tel Mithryn to wither. After Neloth assigns to the Dragonborn the task of finding him a new steward, and after the damaged part of Tel Mithryn is repaired, a third attack causes Neloth to reveal to the Dragonborn his suspicion that the attacks are organised -- the same suspicion that Captain Veleth has about the attacks on Raven Rock. Neloth gives the Dragonborn the task of discovering the source of the attacks.

Walkthrough
The quest is given once you have finished Healing a House or Reluctant Steward.

Ash spawn attack Tel Mithryn. Upon killing them, you are guided to Neloth to notify him of the attack. He states that its not a coincidence that the events that have taken place occurred, and states that someone must be behind the attacks. He give you Neloth's Ring of Tracking which will help identify the culprit. Put on the ring and head out of Tel Mithryn. North-east along the coast of Tel Mithryn is 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. He casts a point and finds that she is in Highpoint Tower.

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 retreats further into the ruins where there are further 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.

Reward
Upon returning to Tel Mithryn, Neloth makes the Dragonborn a member of House Telvanni as a reward. Asking him what it means to be a member of his house, he remarks: "For now, not much. But in a few decades when I return to Vvardenfell, you would be seen as Morrowind nobility. In the mean time, we would keep a bed and chest here for you."

Bugs

 * There is a bug in which the site will glow purple but there is no grave
 * When returning to Neloth to tell him the source of the attacks, he says to "Wait a moment while I cast a more specific divination". However, he won't cast the spell and when the player talks to him again, he asks, "Have you found the source of the attacks?" The dialogue option about the heart stone will be gone, so the player is unable to continue. If this happens you can reload to a time before you speak to Neloth and make sure the quest Healing a House is completed.
 * When you go to rip out her heart it freezes.
 * Found a solution for old friends ask neloth is their anything you can help him with. He'll go on about casting a spell on you which brings up experimental subject quest and cast the divination then the experimental spell.
 * Also found a solution: I disconnected my Ethernet cable from the Xbox let it sign me out, reloaded auto save, talk to Neloth first ask him if you can help with experiments after you receive the mission wait 30 sec talk to him again activating the Old Friends dialog about the Heart Stone then wait another 30 sec walk out towards the lift in the center of the room and wait for Neloth to walk out of the enchanting room he should cast the spell then. Good Luck.
 * Another solution: Attacking Neloth and then sheathing your weapon will bring up the next step in your mission, Kill Idari, and you'll be on your way.
 * There is a bug where you get down to a room with a tripwire at the entrance but the room has no floor. Solution: just leave the building and then head back inside and it should fix the bug.