Tribunal:The Missing Hand

Quick Walkthrough
#Speak with Almalexia who will tell you of Salas Valor. 
#Find Salas Valor in the Godsreach district, dressed like a High Ordinator. 
#Talk to Salas, which will cause him to initiate combat. Kill him, and loot his valuable gear.
#Return to Almalexia once Salas is dead to receive your additional reward.

Detailed Walkthrough
Almalexia
Speak once again to Lady Almalexia in the Temple to receive your next task. It appears that one of Her Hands has abandoned her. Salas Valor is now spreading slander and lies about the goddess to those who will listen, though Almalexia attributes this to madness caused by her very presence. Salas can currently be found somewhere in the streets and sewers of Mournhold.  Your task is to stop him from slandering the goddess by any means necessary.

Salas Valor
If you ask Fedris Hler about the Hands of Almalexia, he will tell you there's a frustrated Hand wandering around in Godsreach. He will look like an ordinary High Ordinator, but his equipment has the magical shine to it. Be well-prepared beforehand, as combat will start as soon as you finish speaking with him. Salas is one of the tougher fights in the game. (See Strategies, below, for details.)

Once Salas is dead, feel free to loot the nice selection of enchanted items from his corpse. However, beware of equipping the cuirass or the helmet, since talking to a High Ordinator with either of these items equipped will cause them to attack you, just like equipping Indoril armor in Vvardenfell. The Hands of Almalexia will not attack however they will state that they restrain themselves at the command of their lady. Likewise, avoid wearing this armor near the regular Ordinators when you return to Vvardenfell.

Of course, you can learn why he has abandoned Almalexia by asking him about being an Ordinator. He will explain that his duties and the god's personality had driven him to leave.

Return to Almalexia
The deed done, return to Almalexia to report. She grieves for the loss of Salas (well, a little) and offers you several possible rewards: 
:#Her Ironskin: Constant 5 point bonus to Unarmored, plus Light, Medium and Heavy Armor. This is implemented as an ability rather than a spell so it will boost skills above 100.
:#Her Guard Against Terror: Constant Resist Paralysis (20%) 
:#Her Reflected Glory: Constant Fortify Health (10 points)
You may also decline a reward.

Strategies
Salas Valor has a lot of Health, heals himself with potions, is magic-resistant (though not completely), and deals a decent blow with his sword (which drains your Strength). Healing potions, magical shields, and enchanted armors and items are highly recommended. So is nimble combat; as with Gaenor, Salas is not someone you want to just stand still and trade blows with. 
During the fight, if you need some time to heal or regroup you can duck inside a building. When you come back outside, Salas will still be there, at the same health level, you left him, as long as you do not use Rest and do not delay (with Wait or by just letting the game run idly) for several game-days (many aspects of NPCs reset after 72 hours of the PC not being in the same cell). If you have completely run away from the fight, to try again after more leveling and training, you may have to reinitiate dialogue to trigger him to attack again, as he may have "forgotten" (and healed up).
Those with a high Sneak skill may find themselves able to make this fight a breeze by cautiously pickpocketing Valor's enchanted Ebony Scimitar. This is an extremely powerful weapon which is the major reason this battle is so difficult. By default, he has Her Darts equipped, which are also powerful (and can blind you) but this allows you to steal his more powerful melee weapon and keep it out of the fight (or at least his side of it), making the battle considerably easier. However, this is not a viable option unless your Sneak skill is in the 70+ range.  Any lower and it will surely fail. Lower than 100 may still also fail. Fortunately, if you have started An Assassination Attempt, a Sneak trainer does business at The Winged Guar, up to level 87.  Note that a very short-term (e.g., 5 or 10 second) Fortify Sneak +100 on Self spell has a low casting cost.  
An inventive but low-risk way to kill Salas is to use a spell or item of Jump (because Levitate does not work in Mournhold), and a number of high-power missiles or ranged spells. Dwarven Darts from the Dwemer ruins in Bamz-Amschend are a good bet, as is firing the higher-end enchanted bolts with the Dwarven Crossbow. Cast the Jump spell and speak with Salas, then jump on top of the nearest building. Feel free to drop all of your armor first, because he won't be able to reach you, and the encumbrance reduction will help ensure that you can jump high enough. Once on top of a building, use your ranged attacks. Take your time lining up the shots, as there is no rush and you are safe. When he falls soon enough, you can jump (with high Acrobat skill) or Slowfall back down, unscathed, to pick up his and your armor.  Note: Sometimes NPCs lapse into panic-and-run-around mode when the PC is far out of reach; it is good to have a strong, ranged, area-of-effect Rally Humanoid spell in case this happens (area because it can be remarkably difficult to hit a panicked moving target).
If you are really having problems with this enemy, you can simply use the Sujamma trick which will make this fight really easy and closable in just a few hits.

Notes
Be warned that even though Almalexia gave you this quest, if you attack Salas yourself, rather than triggering him to attack you through dialogue, the High Ordinators will come to his aid and you will have a Bounty.  (Technically, you could also cast Frenzy Humanoid on him, without initiating dialogue, but this is not worth the Magicka expenditure.)
Be sure to take the enchanted Her Hand's Armor from Valor's corpse, either to wear (but see above about wearing the helm or cuirass around Ordinators) or to sell. It is one of the most valuable suits in the game and is also very effective armor, despite its weight.
Using Calm Humanoid to talk to Salas Valor again is pointless.  He will re-attack you after you talk to him, and if you strike him or cast a hostile spell while the Calm effect is still active, this will result in you getting a Bounty.
If you refuse to take a reward from Almalexia, you miss out on a useful new ability or spell.  It was originally also intended that Almalexia would be offended enough by the rejection that her Disposition would drop by 30 points. In the released version of the game, however, Almalexia is technically a Creature not an NPC, and thus does not have Disposition.  (Some code-patching projects have made her, and some other important Creature-coded figures like Vivec, into NPCs, both to resolve this glitch and so that the living gods cannot be soul-trapped.)
For almost all cases, the best of the available rewards is Her Ironskin, since it is useful for all players, and will raise your defensive skills above 100, a rare opportunity (and it will raise your Armor Rating by more than +5).  Her Guard Against Terror is trivial; you can create an enchanted Constant Effect item with much higher Resist Paralysis than 20% (and should strongly consider doing this with multiple items to have 100% resistance by the time you start regularly encountering Hungers, and higher-level Dark Brotherhood assassins with paralyzing jinkblades).  The 10 points of Fortify Health provided by Her Reflected Glory is a low-end reward for the same do-it-better-yourself reason, and further because 10 points at a high character level is just a drop in the bucket.
It is possible to start this quest without speaking to Almalexia. You may find Salas Valor in Godsreach, and you can kill him on your own, then speak to Almalexia for your reward.  She will offer you an alternative (and also trivial) reward (and only one): Her Tamed Lightning, Constant Lightning Shield (10 points).  Due to a glitch, you will actually receive Her Guard Against Terror, Constant Resist Paralysis (20%), which is one of the two lesser of the rewards normally offered as options.

Quest Stages