Skyrim Dragonborn Old Friends Quest

In Skyrim Dragonborn dlc, the hero will eventually embark on the Old Friends quest for Neloth. This quest is obtained after doing some things for Neloth, notably completing the Heart Stones quest for him and also the Briarheart Necropsy quest. Eventually, Neloth will complain (no surprise there) about "some things" trying to kill him. Neloth will throw in the theory that the dragons are trying to kill him too, but that is quickly refuted by the Dragonborn hero. Neloth will then ask the hero to wear Neloth's tracking ring. This will guide the hero on how to find the source of the attacks and then find Neloth's former apprentice Ildari Sarothril and defeat her in Highpoint Tower.

Wearing Neloth's tracking ring will allow the hero to find the heart stone that is triggering the attacks on Neloth. From Tel Mithryn, head a bit north and a lot east towards the sea and coastline. Around the region near the beach, there will be an ash spawn beside a sarcophagus. When wearing the ring of tracking, the hero can just about see a purplish light emanating from the sarcophagus. Defeat the ash spawn and open the sarcophagus to find a heart stone.

Bring the heart stone to Neloth and he will cast some divination spell that will pinpoint the source and culprit of the attack - his former (and previously thought dead) apprentice Ildari Sarothril.

To find Ildari Sarothril, get to Highpoint Tower. The hero needs to travel northwest from Tel Mithryn towards the centre of the island. When the hero reaches the place, there will be the obvious Highpoint Tower, but no way in from the surface. The hero needs to follow the marker on the map and go into the underpass and into a series of underground passages that will lead to Ildari.

Not surprisingly, there is an ash spawn blocking the door into Highpoint Tower. Defeat the ash spawn. Open the door into Highpoint Tower. The initial stages of Highpoint Tower will be filled with spiders. Defeat them and beware of a lighting shooting soulstone. Knock the soulstone out of its pedestal and continue.

Then the ash spawns return. Defeat them and continue deeper into Highpoint Tower. Reach a series of jail cells and release Niyya. Go through a series of wooden plank bridges and arrive at a large cavern with Ildari positioned at the top. She will call forth some ash spawns and a ash guardian and then run away. Defeat them and chase after Ildari.

Reach an underground cavern complex with more ash spawns that will erupt from the soils. Eventually, the hero will reach a large chamber with a blackened ring on the ground. If the hero steps into the ring, he will be pelted with fireballs. Raise your head to see Ildari and two fireball soulstone shooters at the top.

Go into sneak mode and turn left and sneak upstairs through the stairway. Emerge on the top and notice Ildari across the bridge on the other side with the fireball soulstones. Use Serana as the tank against the fireballs and then sneak arrow attack Ildari. When Illdari is down, sneak roll across and activate the lever that lies below the bridge to deactivate the fireballs. Then rip out Ildari's heart.

Go report to Neloth that Ildari has been defeated. The reward is a staff of repulsion.