Tribunal:Crimson Plague

Quick Walkthrough
#Travel to Gedna Relvel's Tomb.
#Talk to Mehra Helas.
#Talk to Nerile Andaren.
#Deliver a potion of cure common disease to Geon Auline.
#Return to Nerile Andaren.
#Deliver a potion of cure common disease to Athelyn Malas.
#Return to Nerile Andaren.
#Attempt to enter the Temple Sewers and talk to Shunari Eye-Fly.
#Return to Nerile Andaren and receive a scroll from her.
#(Optional) Find Shunari Eye-Fly near the end of the Temple Gardens.
#Travel to Gedna Relvel's Tomb in the Temple Gardens.
#Kill Gedna Relvel.
#Return to Nerile Andaren for your reward.

Detailed Walkthrough
Starting the Quest
Opening the level 50 locked door and entering Gedna Relvel's Tomb will trigger a foreboding message about an unusual smell and presence inside the tomb. However, searching the tomb will reveal that it is empty. After returning to the city, you may speak with Mehra Helas in the Temple Courtyard. Asking her about the latest rumors will reveal that there is a new sickness going around. As the cause is still unknown, she refers you to the Temple healer, Nerile Andaren. 

Delivering Medicine
Nerile Andaren believes that the sickness has something do with infected rats. While she claims the rumors of an epidemic have been exaggerated, she would like to take quick action healing the infected nonetheless. Nerile's first task is to deliver a potion of cure common disease to Geon Auline at his manor in Godsreach.

Upon returning to the Temple after the delivery, however, you will discover that the Hall of Ministry has been infested with infected rats. Nerile now believes that she underestimated the situation; infected citizens are showing up at the temple for healing but are not being admitted inside. Nerile's next task is to deliver a cure disease potion to Athelyn Malas, who can be found on the exterior Temple steps begging for help. 

Meanwhile, Nerile discovers through her research that the recent illnesses have been caused by the Crimson Plague. The disease was supposedly eradicated in the Second Era, but has mysteriously reappeared. Something must be done quickly because the disease spreads rapidly. She asks you to check the Temple Sewers to see where the rats are coming from, as this is likely the source of the infection.

Investigating the Source
When you enter the Temple Basement you'll find Shunari Eye-Fly standing over an unconscious High Ordinator. According to Shunari, she had her eye on Gedna Relvel's Tomb for some time, but could not figure out how to open the door. She watched you exit the tomb and entered shortly after, hoping to find some leftover treasure. Unfortunately, she became sick and fled to the Temple for aid. Upon clambering out of the sewers, she discovered the guard, who she knocked unconscious to prevent him from raising the alarm. Now that you've found her, however, Shunari promises to reveal what she found in the tomb if you cure her, but there is a catch: she fears you will poison her if you use a potion, so she will only agree to be healed by spellcasting. The options are either to return to Nerile for a scroll of Cure Common Disease, or else to use a spell of your own. 

Once Shunari has been cured, she will explain that the tomb's inhabitant, Gedna Relvel, is a lich. When you opened the tomb, you unknowingly released the plague on Mournhold. Be sure to ask Shunari about the secret entrance that she saw Gedna use to access her abode. It is impossible to activate the stone doors leading to Gedna without first asking Shunari about it.

The Tomb
thumb|Gedna Relvel
Locate the set of stone doors adjacent to the ladder at the tomb's central juncture. Simply standing in front of the doors will cause them to open, as there is a stone trigger on the floor that keeps them shut. A few Skeleton Champions can be found in the passage just through the doors, as well as a couple more in the room with Gedna. Be sure to recover your strength before engaging with Gedna, however, as she is one of the most difficult opponents in the game. 

Some strategies for defeating Gedna are listed below. The lich is highly magic resistant and casts a very powerful destruction spell that can cause as much as 600pts of elemental damage per cast. Gedna also possesses a unique robe that may be of interest; however, its enchantment is of dubious usefulness. 

Return to Nerile once the Lich is dead to receive her congratulations and the new power, Grace of Almsivi (which may be used once per day to Restore Health 30 to 80 pts on Self, Restore Fatigue 60 to 100 pts on Self, and Cure Common Disease on Self).

Strategies
Preparation: While Gedna's destruction spell deals 100-200pts each of Fire, Frost and Poison damage (accounting for 300-600pts of elemental damage per cast), Dark Elves, Redguards, Nords and Argonians will either be majorly resistant (75%) or completely immune to one of the three spell effects due to their base resistances. Casting spells or imbibing potions to resist the other types of elemental damage may be worthwhile if your total health is low enough to put you within risk of being killed in one hit by Gedna. Alternatively, items that provide protection from spells, such as the Ring of Equity, will allow you to survive.
Ranged Weapons: The columns in the tomb can block Gedna's destruction spell, allowing a user of Marksman weapons to move in and out of cover while firing at her. This can safely and reliably wear down her health, if timed correctly to avoid each cast of her spell. A high Speed attribute or a high-point Levitation effect can make the process easier.
Sneak: Gedna's dialogue prompt is scripted to appear when the player gets close to her. However, with a high sneak skill and the use of chameleon, it is possible to remain undetected after closing the dialogue box and to sneak behind Gedna for a critical hit before she can cast her buffs and damage spells. Buffing your damage as much as possible for this one critical hit can prove extremely effective.  
Stun or Stagger: Staggering Gedna to stop her from spellcasting is likely the most useful strategy to defeat her. High base damage weapons are ideal, although be careful with enchantments as Gedna is immune to frost, poison and shock. If the Tribunal main quest has been completed before battling Gedna, Trueflame is a particularly useful weapon for its high base damage, fire enchantment, and quick striking speed.   
Fortify Skill, Fortify Strength, and Fortify Endurance effects can help your attacks to overwhelm Gedna and disrupt her spellcasting and health regeneration.   
Spellcasting: Direct spellcasting may prove too slow for this fight. Consider utilising enchantments or buffing your stats before getting close to Gedna. Moreover, it should be noted that, while Reflect potions or enchantments can be useful for deflecting damage, you should not use these for dealing damage because Gedna is completely immune to Frost, Poison and Shock damage, and has a spell which grants her 600% resistance to magicka.

Notes
If you accidentally use or misplace one of the potions Nerile Andaren gives to you, you can simply buy another potion of cure common disease (including from Nerile herself, as she is a healer).
Gedna Relvel's stat-boosting script to match the player's level is broken. Therefore her normal level is 40 and her normal health is 700. However, there are conflicting reports whether the broken script may occasionally cause an 800x health buff that can make killing Gedna virtually impossible at higher levels. See the discussion tab of her page for more information.
In the unpatched version, you must speak with Mehra Helas about Latest Rumors before the An Attack on Mournhold quest. Afterwards, she will only mention the Attack on Plaza Brindisi Dorom and that you were there.
Killing the disabled High Ordinator has no consequences aside from a complaint from Shunari Eye-Fly. You may exploit this situation if you are after his armor or ebony scimitar, although these can be acquired elsewhere (such as in the course of The Thief quest).
You can heal Shunari Eye-Fly yourself with a Cure Common Disease spell; this also revives the guard. Doing so will automatically advance the quest without Nerile's intervention.
If you kill Athelyn Malas or Geon Auline, Nerile Andaren's disposition towards you drops 50 points.
Doing this quest alongside The Shrine of the Dead can save having to clear the sewers twice. 

Bugs

Quest stage 70 cannot be obtained, and may be bugged considering its reference to Goval Ralen who is not related to this quest. 

Quest Stages