You get this quest by visiting Darklight Tower. Upon entering, open the door in front of you to see a witch named Illia KILL another witch. You'll now talk to her and learn she is trying to escape this evil lifestyle, but just now decided that she must stop (read: kill) her mother instead. She'll ask for your help, so agree to help to start the quest!
Illia will be your follower now. There's a door on this ground level you can pick for a chest, then head up the stairs to slay some spiders. Continue on to find a witch in the next room. Kill her and use the Alchemy Lab if you wish, then head up some stairs and kill another witch and a troll (the upstairs here has some goodies on a shelf, but watch the floor trap). Keep on going but look out for webbing on the right hand wall as you can cut it away to reveal a chest. Further in grab some potions from a shelf and then pull the lever nearby to clear the way. Run up the stairs and assault the hagraven like Illia said there would be (kill a spider and a skeever as well). Check the table nearby for a soul gem and "2920, Sun's Dawn, v2", an ILLUSION SKILLBOOK. Proceed into Darklight Chambers now with Illia.
Proceed upwards and take out the two witches. Illia will head to a door, but it will be locked with a MASTER lock. You could unlock it, but for us mere mortals we will have to take the other door up to a room with a hagraven in it. Quickly close the gap between you and the hagraven (or start killing it quickly) as it fires off powerful fire magic. There's a hag here as well. Kill them both and loot the hagraven for the tower key. Be sure to grab the chest from the tent and check a bookshelf for potions as well. Return to Illia and unlock the path. Continue on and kill another witch, then grab the potions and chest in the room (and use the arcane enchanter if you wish).
Head upstairs some more and take out the two spiders and Hag. Check the table here for a DESTRUCTION SKILLBOOK called "Mystery of Talara, v3". Continue heading up and at the top Illia will stop you and tell you her mother is just through the next door, but she has a plan. She wants you to act as the sacrifice and play along to distract her mother. Head through the door back out into Skyrim now.
Out here, play along with her mother and sit in the chair. The mother will start to "prepare" you while Illia strikes from behind. A pathetic strike, I might add, so quickly get up and finish the job! Afterwards, she'll thank you. Loot the mother for an expensive staff and loot nearby for a BIG chest and a DESTRUCTION SKILLBOOK called "Horror of Castle Xyr".
You can now ask Illia to be your follower, if you wish, as she really has nothing better to do. You've seen her in battle so you know how powerful her ice magic is. You'll now be done with this quest.