Morrowind:Lord's Mail (quest)

Quick Walkthrough
#Speak to Varus Vantinius.
#Enter the caves through the hidden door in the shrine or the underwater entrance.
#Defeat Furius Acilius in battle.
#Return the Lord's Mail to Varus.
#Travel to Sadrith Mora and ask around town about Draramu Hloran.
#Go to Abanabi.
#Kill Draramu Hloran.
#Return Chrysamere to Varus.

Detailed Walkthrough
Lord's Mail
Once you've reached the rank of Knight Protector in the Legion, you can perform this quest for the Knight of the Imperial Dragon, Varus Vantinius, found in the Grand Council Chamber in the Duke's Castle in Ebonheart. Varus informs you that in order to become a Knight of the Garland you must retrieve two missing artifacts. The first is the Lord's Mail, which disappeared from the Imperial Commission on the ground floor and to which you are given a key. To start your investigation, talk to the Imperial Magistrate Rufinus Alleius, who suspects the banished legionnaire Furius Acilius, who was talking about the armor and the caves below the castle before he left.

A closer inspection reveals how the thief got in, via a well-hidden and locked (95) door located in the Imperial Shrine itself (Someone forgot to wipe their feet).  If you are a skilled enough thief, this should pose no problem to you, but you can also find the Worn Imperial Key in Acilius' room, in the southwest portion of Imperial Guard Garrison, underneath his pillow. The secret door leads to a small cave where Acilius is trying on the missing armor. After a short talk, he will attack you; kill him in a tougher-than-average fight and retrieve the armor. There is more loot in the cave; see the notes. Return the armor to Vantinius (don't worry, you'll get it back soon).

If you would rather not risk getting caught, you can walk from the main entrance of Ebonheart along the path. When the path turns right, continue over the hill. This should bring you to a sandy area. Swim through the water to the tip of the peninsula. Under the water, you should reach a door leading to the underground caves. When you enter, kill all of the rats, then kill Furius, and you can leave out the way you came. If you still want to get into the room with the secret door, swim through the pool of water by his camp, and you will see a floating crate. Inside is the key to the secret wall door.

If you refuse to give Varus the armor twice, he will get offended and attack you. This will not have any legal consequences or expel you from the Legion, but it will prevent you from dueling him later for the last rank.

Chrysamere
The second artifact to recover is the legendary sword, Chrysamere, which was stolen by another knight. Spies have learned that the sword is in the possession of the sorceress Draramu Hloran near Sadrith Mora. If you travel to Sadrith Mora and ask around town, you'll quickly learn that everyone is scared to death of Hloran and won't speak a word of her to you. You'll have to boost an NPC's disposition to at least 75 in order to get one to spill the beans. Hloran can be found in the cave Abanabi, southwest of town on a large island. Hloran, among other things in the cave, can be found in a slightly concealed side corridor. Note that there are many leveled Daedra of random sorts in the cave. Once you kill her, return the sword to Vantinius. If you refuse to give him Chrysamere, he will attack you.

Report back to Vantinius with the two artifacts to receive the Duke's Guard Armor.

Notes
Picking mushrooms near the short east path will reveal a Glass Jinkblade (worth 6500 gold). 
If you levitate up to the left of his campfire, you'll find a shelf with a Glass Frostsword leaning against a rock, a Pilgrim's Lantern, a few sacks, a crate and a locked chest.
Instead of risking death fighting Furius Acilius, leave the cave through the underwater door after combat with him begins. Enter Ebonheart, and rest for several days. Then enter the cave again, and if you timed it right, Furius Acilius should be swimming underwater. He shouldn't attack you if you've waited long enough. Then just swim around him, and wait for about a minute. You will see him drown and float up. Now the Lord's Mail is yours for the taking.
The Worn Imperial Key in the floating crate is only good for opening the secret door. It will not open the regular entrance.
If you talk to Furius Acilius before attacking him, he will say that it must remain a secret, and to ensure that, he will kill you. You get a "Goodbye" prompt, and he will then attack you.
You do not need to be Imperial Legion to get and use the Lord's Mail (though you cannot trigger the quest for it and Chrysamere if you are not a Legionnaire of the proper status, other than perhaps with the Eiruki/Chanil-Lee glitch mentioned above). Simply swim to the cave where Furius is hiding, or get the secret-door key from Furius's garrison bedroom, and go find him.
Queststage 30 and 70 cannot be obtained in-game.

Bugs
 

Quest Stages