Online:The Soul-Meld Mage

Quick Walkthrough
# Talk to Gadris.
# Talk to Zur.
# Collect Zur's notes and tincture.
# Enter the laboratory.
# Squeeze through the rubble and find Gadris' notes.
# Gather the items and catalyst, and forge the antipodal rods.
# Meet Gadris at the Overseer's Atelier.
# Find the power crystal.
# Activate the machine.
# Take the Light of Meridia and leave the laboratory.

Detailed Walkthrough
 
Two souls in one body
On a platform near the Vile Laboratory you'll find a dunmer named Gadris who's seemingly talking to himself. Upon entering conversation, however, it turns out his soul is melted together with a khajiit named Zur's soul in some daedra experiment. When the conversation exits Gardis will transform into Zur, after which you need to start conversation once more. Zur will explain that to help, you need to get to a dwarven device that's hidden behind some rubble. Luckily, Zur can make a potion that allows you to pass but you need to gather his notes and tincture. He also explains you can't be teleported inside due to their lack of energy.

Obtaining the notes and tincture
Go over the south bridge leaving the platform. The first note Transmutation of Living Creatures is just around a wall near a bookcase guarded by a single zombie. The second note Transmutation Potion Recipe can be found a little bit more south on a stone fence. Follow the path behind the fence going up and follow that path north. There will be a stone building, inside, you can find Zur and the tincture. Optionally you can talk to Zur for a bit of extra conversation. Grab the tincture, and go to the entrance of the Vile Laboratory. The entrance is on the lower path next to the stone fence you previously visited. Enter the laboratory. Inside you see Gardis transforming into Zur again. Talk to Zur.

Crafting the antipodal rots
Wait for Zur to make his potion. Zur is done, he changes into Gardis again. Talk to Gardis. He tells you that once you're inside you should look for his notebook and follow its instructions on crafting antipodal rots. After an explanation of what that means, the conversation will end and you will automatically drink the potion causing you to change into a monkey. Use the broken pipes to maneuver around the room and to avoid the burning plateaus. Once you enter the metallurgy room you automatically return back into your regular self.

Read Gardis's notebook located in the middle of the room and collect the refined void salts, the ebony ore, and the Crystalline Essence Matrix from three of the corners of the room while killing the zombies and dwemer automation in the room. Return to the middle of the room to read the Catalyst Note. Once you approach the middle of the room, Zur will come in to say a few words. Go through the door on the south side. A flesh atronach named Phazkad protects the room. Kill him and grab the corrupted blood, then return through the door you came through. Now, go to the Force Furnace use it and the nearby weapon anvil to craft the antipodal rots. After this, go through the door on the north side of the room.

Unmelding the souls
Go through the hallway killing the dwemer automations on your path and talk to Gardis. He's rather tired but will reveal to you that only one of the two will be able to survive the unmelting and that the choice is up to you who that is. He tells you that he's grown rather fond of Zur, and can't advise because they both deserve equally to survive. He also tells you to look for a glowing crystal as it's used to power the device. Go inside and kill Overseer Xvarik who's guarding "the glowing crystal" which is, in reality, a light of Meridia. Grab the light of Meridia and walk to the other end of the room. There Gardis will appear, soon changing into Zur. Talk to Zur, place the crystal and the two rods. Now comes the irreversible choice. Save Gardis or save Zur.

Saving Gardis
To save Gardis, use the left switch. Talk to him, grab the light of Meridia, and go outside. Talk to Gardis again. After this Gardis will mention he misses Zur and that he's ashamed that he judged Zur zo negative and tells you he was a good person. He will also tell you that he and Zur found out the drawven device inside and the Molag Bal's Planemeld use the same principles. After this the quest will be completed and he will create a portal to the Hollow City.

If Gardis is saved, he will attend the war council if it hasn't taken place yet and join Gabrielle Benele's team during the final assault where he will give you a light spell for the dark chamber. Otherwise, he can be found in the dark alchemy labs in the Hollow city after the quest is completed.

Saving Zur
To save Zur, use the right switch. Talk to him, grab the light of Meridia, and go outside. Talk to Zur again. Zur will tell you he feels lonely and misses Gardis, saying he used to dislike him but now feels like he misjudged him and is positive about Gardis. He will also tell you that he and Gardis found out the dwarven device inside and the Molag Bal's Planemeld use the same principles. After this the quest will be completed and he makes a portal to the Hollow City.

If Zur is saved, he will attend the war council if it hasn't taken place yet and join Gabrielle Benele's team during the final assault where he will give you a light spell for the dark chamber. Otherwise, he can be found in the dark alchemy labs in the Hollow city after the quests is completed. Zur will note that he keeps his notes much more tidy thanks to his time merged with Gardis.

Gallery
 

Notes
After completing this quest, the Dark Alchemy Lab in The Hollow City will unlock, and Gadris or Zur will be found living there.
 

Quest Stages