Oblivion:The Shrine of the Crusader

Quick Walkthrough
This quest only becomes available once you have the Knights of the Nine official plug-in
# Receive a vision from Pelinal.
# Visit Vanua and discover Sir Amiel's body, the journal found on his body starts the next quest. 
# Retrieve the Helm of the Crusader to finish the quest.

Detailed Walkthrough
The Vision
After you have completed the Pilgrimage, you will receive a vision. Upon praying at the final wayshrine, you will find yourself transported to the skies above the Imperial City, appearing before the Divine Crusader, Pelinal Whitestrake, who reveals more details about himself and Umaril, and how Umaril has returned. Because of Umaril's alliance with Meridia, when struck down, rather than being killed, Umaril's spirit is returned to the waters of Oblivion to slowly regain his strength and return once more. The only way to defeat Umaril once and for all, he explains, is to reclaim all of the relics and seek the Nine's aid. Although he does not know of the relics' current whereabouts, as they have exchanged hands many times since his death, he does have a guess as to where one could be. He suggests searching his lost shrine, the Ayleid ruin Vanua, which is where he last saw the Helm of the Crusader. Because the ruin is well off the beaten path and guarded by numerous beasts, Pelinal is fairly certain the Helm has not yet been retrieved and is still there. If nothing else, the shrine may provide a few clues as to how to proceed.

Sir Amiel's Legacy
No sooner than you are released from the vision does your journal update, and a map marker points to Vanua's location.  You are also made a member of the Knights of the Nine faction, with the rank Knight Errant.

Vanua is located near the Imperial City, in the area where the Upper Niben meets Lake Rumare. Swim underwater in that location and you will find the submerged entrance to the ruin. The way is fairly straightforward, akin to the layout of a standard Ayleid ruin, though at some points you will need to enter rocky, musty caverns accessible through holes in the walls to bypass cave-ins. Within the second zone of the ruin, the Shrine of the Crusader, you will find the body of the fallen knight Sir Amiel, armed with a Fine Steel Longsword and a Knights of the Nine Shield. Search his body to find his journal and ring. Reading the journal yields more information about the fallen knight, learning that he was the leader of the Knights of the Nine, a faction dedicated to reclaiming the relics just as you are now. It details how internal strife within the order, sparked by greed to use the relic's power for personal gain, led to the ultimate disbanding of the Knights. However, the headquarters of this faction, the Priory of the Nine (located in the West Weald, southeast of Skingrad near the Elsweyr border), still houses the Cuirass of the Crusader. Sir Amiel's ring unlocks a secret area within the Priory where the Cuirass may be found. Before you can reclaim it, though, there is still the matter of retrieving the Helm.

The Helm of the Crusader
Continue through the Shrine of the Crusader on to The Lost Catacombs, a brief but monster-infested path which will loop back up into the upper-half of the Shrine of the Crusader. There you will find the Helm of the Crusader on an altar atop a ledge overlooking the body of Sir Amiel. Take it to finish the quest.

However, there are a number of ways of reaching the Helm without going through The Lost Catacombs, including:
With a good Acrobatics skill or the Boots of Springheel Jak, you can just jump up over the railing to the upper-half of the section.
Even with moderate Acrobatics, you can still jump onto one of the half-broken pillars, and then onto the side wall, where you can jump again around the pillar and then over the fence.
Finally, regardless of your Acrobatics, if you're fast enough, you can run along the very thin ledge near the hole you initially entered the room from and then jump over the fence from there.  Do not stop or even pause to think about what you're doing, though, or you will likely fall off. It is best done in one swift movement.
You can also use the paint brush trick here.

One thing to note about jumping over the fence is to make sure you've grabbed Sir Amiel's ring first, to open the door in the Priory later on. Otherwise, you will have to return here to retrieve the ring in order to proceed.

Whatever method you choose, once the Helm is in your possession, exit the ruin and proceed to the Priory of the Nine, where the next relic lies.

Notes
After receiving Pelinal's vision to start this quest, the game decides whether you will receive the light or heavy versions of the crusader's relics. If your Light Armor skill is higher than your Heavy Armor skill at this point you will receive light armor versions of all armor pieces, otherwise you get all heavy armor versions.

Bugs
Sir Amiel's Key will remain a quest item even after completing this quest, meaning you can never remove it from your inventory.
 You can fix this yourself by entering setquestobject xx000FC0 0, where xx is the mod index of the Knights of the Nine plugin.


Journal Entries