The Blood Ritual
You can only send one mage into the Fade. Now it can be your character, Morrigan, or Wynne (healer from the mage quest). It seems like Morrigan slightly disapproves of being drafted for the rescue mission, so keep that in mind. I pulled this off with Morrigan, for the record.
Regardless of who you pick, you’ll be in a blurry and odd world once the loading is done. It’s really hard to give directions too. You basically just have to wander forward, find Conner, and then go back through a fade portal. You should be fine. Walk along the path and hit the first portal. You should be able to find Conner. He’ll turn into the demon and attack you. This is a weak version though, so just hit her with a spell or two.
Move to the next portal and go through. You should find another version of Conner here. Just kill the demon when she spawns and then turn on her lesser rage demon. You can lay down paralyzing glyphs, if your mage knows the magic. That way the demon will be stunned as soon as she spawns. There's just one more that spawns, and she'll spawn two demons. After she dies, you can go through the portal to reach the end.
The demon should offer a trade to you. This will only happen if you sent your character through. Even Morrigan refuses to hear the demon’s offer. There’s not necessarily much of a benefit to a fight, so you might want to think about the deal. There’s not really a negative to it and this is the only way to get the blood mage specialization for your character.
If it comes to a fight, then be ready to fling a lot of spells and pop a lyrium potion. Besides a basic bolt spell, she uses Horror, Drain Life, and Spell Shield regularly. Just start your attack and fling some spells at her. Winter’s Grasp and Cone of Cold both froze her for me. Similar spells should have good effects. Just hit her a few times to grind her health down and get her to warp out.
She’ll spawn a bunch of copies around the circle. If you attack a copy, it will fade and restore her health. She can get a fair bit back if you hit all of them. You should be able to see the real version, because it will be surrounded by a bit of cone of light from where she teleported out. When you hit the real one, she’ll attack again. You should be able to rinse and repeat your spell series. She doesn’t seem to unleash too many attacks at this point, so you should be able to just focus on attacking.
After the second copying, she should get more defensive and use her shield spells more. Just keep plugging away and you should be able to kill her. Once she dies, you will return to the regular world and see the end.