Find the source of the attacks
Notice! This mission will be available only after you complete Healing a House and Reluctant Steward!
The repeated attacks of Ash Spawns are becoming hard to find accidental. Master Neloth thinks that someone might want to kill him and that's why he's sending the monster onto Tel Mithryn. If you want, you can help the mage. He will give you Neloth's Ring of Tracking thank to which you will be able to locate where the monsters are coming from.
Put on the ring and head out of Tel Mithryn. Head northeast (a couple hundred steps from the tower) an you should come across a small cemetery. Head closer to notice a pink glow. Move closer to it and fight with an Ash Spawn. Take a look into the glowing coffin and take the heartstone and staff. That should be enough for Neloth.
Talk to Neloth
Inform the mage of your discovery and he will tell you of a certain failed experiment conducted on his former apprentice Ildari Sarothril. Master Neloth implanted the girl with a heartstone in place of her real organ - just like Forsworn Briarhearts do. Unfortunately the procedure didn't go as planned and the woman died. Since her body isn't in the coffin, she must have raised from the dead and wants take revenge on her former teacher.
The undead woman hides inside Highpoint Tower - halfway between Raven Rock and the Ashfallow Citadel. You will be attacked by two Ash Spawns at the very entrance and it won't be any easier inside.
There is only one path you can follow inside the tower - downwards. Go down the round stairs until you reach a small cave. The path will branch here - if you turn left, you will come across lots of spiders and reach a cave with treasures - on the walls you will find various lodes (including sapphires and rubies). After filling up your pockets (you will need a pickaxe!), turn into the right branch of the corridor. Pick up the soul stone which drives the trap and defeat a few more Ash Spawns. In the further part of the dungeon you will come across an identical setting - a trap and enemies.
When the corridor ends, you will enter a small hall with two traps which you should try to avoid - one is magical and the second consists of falling boulders. In the next room you will see a fork in the road - on the right there are prison cells (you can free a prisoner there) and on the left a passage to the further part of the dungeon. Saving the prisoner isn't required, but it's always good to help a fellow soul.
Follow the corridor while keeping an eye for more enemies and precious stones (apparently there used to be a mine here). Eventually you will reach a large cave in which Ildari summons the Ash Spawns and Ash Guardians. Kill the demons and the undead Elf will run away - head onto the wooden construction and chase her.
Note that the shelves around contain quite a lot of alchemic ingredients and magic artifacts - it's worth to take care of them.
The further path is rather straightforward - more corridors and monsters. Eventually you will reach a vast room with two fire traps - they will note you even if you're a master at sneaking. Quickly run through the room and head onto the second floor. That's where Ildari resides. Before attacking her, switch off the pillars of fire - they are more dangerous than the enemy herself.
Fighting the witch isn't very hard - although she knows some useful spells (mainly shock and fire-based), her low health makes her an easy victim. When she falls down to the ground, approach her corpse and tear out her heart (!).
Before leaving the room, be sure to search it thoroughly - in the corner you should see a treasure chest and on the shelf some valuables. If you read the witch's notes, you will learn that she's the one who resurrected General Falx Carius (March of the Dead).
Talk to Neloth
Master Neloth, thankful for solving his problems, will let you join the house of Telvanni, which is a very big distinction. Additionally you will receive a powerful staff, useful when fighting with a couple enemies at the same time. Still, it means a lot less than being a part of the ancient and renowned family - who knows what benefits it might bring you in the future?