Morrowind:Yagrum Bagarn and Wraithguard

Quick Walkthrough
#Kill Vivec, and retrieve the Unique Dwemer Artifact from his corpse.
#Take it to Yagrum Bagarn in the Corprusarium in Tel Fyr.
#Find Kagrenac's Journal and Planbook in Endusal and Tureynulal.
#Bring the books to Yagrum, and wait a day while he studies them in order to harmonize Wraithguard to your magicka signature.
#Equip Wraithguard, and be prepared to lose 200-225 Health, permanently.

Detailed Walkthrough
God-Killer
So, you killed Vivec, and you still want to complete the Main Quest?  Don't worry, all is not lost.  The Back-Path is much more difficult, but still possible.  First you must retrieve the Unique Dwemer Artifact from his divine corpse.  Nobody will tell you what to do with it.  You must find out on your own.  It might help to look at the scrolls found in the alcoves inside Vivec's Palace.

The Last Living Dwarf
Yagrum Bagarn is the last of the Dwemer, and he can be found in the Corprusarium in Tel Fyr.  If you have done much of the Main Quest, you would have met him during the Corprus Cure quest.  If you speak to him with the Unique Dwemer Artifact in your possession, he will recognize it as Wraithguard, and offer to help you activate it.  Ask him about "mythopoeic enchantments" to hear his advice on the matter.  Note that you must have a Reputation of at least 20 in order to receive his aid.  You can show him ''Dagoth Ur's Plans and the Plan to Defeat Dagoth Ur'', both found in Vivec's Palace. If you do not have them, he will still help, if you insist.

Kagrenac's Books
Yagrum will ask you to retrieve two books written by Kagrenac, who helped to construct Wraithguard.  They can be found in Endusal and Tureynulal, respectively.  These two ancient Dwemer Ruins are located on either side of the Dagoth Ur Facility in the Red Mountain region.  They are home to many powerful Ash Creatures, including the Ash Vampires Dagoth Endus and Dagoth Tureynul.  Once you have found the Journal and Planbook, return to Tel Fyr and present them to Yagrum Bagarn.

Harmonizing Wraithguard
When you show him the books, Yagrum will ask you to give him a day to study them.  Return in 24 hours (or just wait), and speak to him again.  He will have Wraithguard ready for you, but he will give you a strong warning.  Equipping the artifact will cause a permanent loss of 200-225 maximum Health.  He will recommend that you use a Fortify Health; however, this is not practical. Fortifying your health only fortifies your *current* health and doing this will not avoid the loss to your *maximum* health. Once the spell or potion wears off, your health will then be reduced by however much you fortified it, and if this brings you below 0 you will die.  Likewise for wearing Constant Effect enchanted items.  If you remove them, or if they are damaged to the point of breaking, you may find yourself dead rather quickly.  The safest thing is to have more than 230 health to begin with, and probably a lot more, as wandering around Red Mountain with only 20 maximum health is likely to be somewhat hazardous.  Fortunately, this loss of health is one-time only, and if you survive, you may equip and unequip the Wraithguard as many times as you wish without worry.

Notes
Vivec has the largest soul in the base game. If you plan on killing him and capturing his soul, you will need to obtain Azura's Star first, as it is the only soul gem capable of storing his soul.
Wraithguard is only considered necessary to hold Keening and Sunder, but this is not a hard requirement. Without the Morrowind Patch Project, equipping the jury-rigged Wraithguard will deal massive, permanent health damage. As this is highly undesirable, Xbox and other non-MPP users may instead opt to forego it entirely. The "Mortal Wound" effect from holding Keening and Sunder without Wraithguard deals large (25-120?) amounts of damage per second, but they need not be held long and this is survivable with enough health.
Obtaining Wraithguard in this way does not trigger Wulf to spawn in Ghostgate, meaning it is impossible to acquire his lucky coin unless you have the Bloodmoon expansion.

Bugs
 
 
 
 

Quest Stages