Oblivion:The Sword of the Crusader

Quick Walkthrough
This quest only becomes available once you have downloaded the Knights of the Nine official plug-in.
#Discover Underpall Cave and Sir Roderic's corpse.
#Defeat the wraith of Lord Berich Vlindrel and receive the Sword of the Crusader.
#Lift the curse from the sword at the Chapel of Arkay in Cheydinhal.
#Return to the Priory of the Nine.

Detailed Walkthrough
Vlindrel Monastery
Upon arriving at Underpall Cave you will automatically receive this quest and an update that says to find Lord Vlindrel and recover the defiled sword.
Note: Depending on whether you accepted Sir Lathon into the Knights of the Nine during the previous mission or not, he may be accompanying you during this quest.
Follow the quest marker through the cave and into a buried fort to the door for the Underpall Reflecting Chamber.  There are five zones total to Underpall Cave, however you need only make your way through three to find Lord Vlindrel; the latter two simply loop back around to the first zone. From Underpall Cave, enter the door in the round tower that leads to Underpall Keep, then turn left and left again and follow the corridor to its end to find the door to Underpall Reflecting Chamber.

Knight of the Cursed
Once inside the Reflecting Chamber you will find two paths, both leading to Lord Vlindrel. The left path is shorter and you will find the body of Sir Roderic on it.  The right path, while longer, is free of enemies and flanks around to a semi-hidden entrance on the very far side of Lord Vlindrel.  Regardless of which you choose, you'll find yourself in a room with Lord Vlindrel's Wraith.  After the battle you will be able to reclaim the Sword of the Crusader.  The sword has been tainted by evil, just as Sir Lathon had explained. You receive a quest update stating that you should reconsecrate it at the Chapel of Arkay in Cheydinhal.

Shadow to Light
Make your way to the chapel, and upon entering you'll find it being attacked by Umaril's Aurorans.  After the battle, pray at the Altar of the Nine.  The sword will be removed from your inventory then put back.  You receive a quest update stating the sword has been purified; you have now recovered all of the relics, and you should make your way back to the Priory to discuss with your Knights what to do next.  When you return to the Priory of the Nine, Sir Thedret will approach and speak with you, officially completing the quest and automatically starting the next.

Notes
Try to lure Lord Vlindrel away from the reflecting pool before killing him, otherwise he will drop the sword in the water, making it harder to retrieve.
If you equip the Sword of the Crusader prior to having reconsecrated it, you will be afflicted with ''Lord Vlindrel's Curse''.  The curse will remain even after you unequip the sword; the only way to remove the curse is to purify the sword at the Chapel .  The curse has some nasty effects: you are afflicted with Stunted Magicka (your magicka will no longer regenerate with time), and are given 100% weaknesses to all of the following: fire, frost, shock, magic, and poison.

If you have not yet received a reference to start Restoration Master Training with Oleta, you may want to delay starting this quest.  During this quest, Ohtesse, the advanced restoration trainer in Cheydinhal chapel, will always die.  She is the last person who can possibly provide the restoration training reference (Marz is already dead, and her replacement, Beem-Kiurz, provides training but not the reference).
 

If you want to capture the wraith of Lord Berich Vlindrel's soul, note that he is considered to be a creature and has a leveled soul.
This is in contrast to the souls of the eight Knights of the Nine fought during the Priory of the Nine quest, whose spirits are considered to be NPCs and thus, their souls are Grand Souls.

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