The Only Cure

Background

"A Khajiit named Kesh has offered to help me commune with Peryite. He asked me to acquire a deathbell flower, flawless ruby, silver ingot and some vampire dust."

―The Dragonborn

Overview

Click here for a video tutorial on how to complete this quest.

Given at the Shrine to Peryite by a Khajiit named Kesh. He tells the player to speak to Peryite, they must:

Obtain a Silver Ingot
Obtain a Deathbell flower
Obtain some Vampire Dust
Obtain a Flawless Ruby.
Give all of the acquired items to Kesh
Inhale the fumes
Kill Orchendor
Report Orchendor's death to Peryite

Walkthrough

Collecting the ruby is by far the hardest part of the initial task. There are a few methods you can try in order to get one for this quest.

You can go to Mistveil keep in Riften and talk to Harrald who will give you the job to retrieve his sword from Balimund. He will then reward you in a random gem combination, if you don't get the flawless ruby on your first try just reload the most recent save and return the sword again until you get it.

A Flawless Ruby along with other precious gems can always be found in a broken boat in the river southwest of Sky Haven Temple. They are sitting in the boat under the seat near the strongbox but not in it. (Confirmed)

Another alternative method can be to mine for one randomly at the Sanaurach Mine. It is advisable to save prior to attempting this method, same as retrieving Harrold's sword in Riften as mining a gemstone is random. This mine is in the town of Karthwasten, to the right up the slope as you face out from the front door of the Karthwasten Hall. Karthwasten is to the southeast of the Shrine to Peryite, and you can see its marker from the Shrine. Once you get inside the mine, the path will split at the bottom to the left and right. Take the path to the right, go up the wooden ramp, keep left and go up another wooden ramp. At the top there are two Silver Ore veins on your left. If you keep going in the same direction and go up another wooden ramp, there are three veins on the left there as well. Remember to save your game before the first two, then mine all five veins at once. If you do not get a Flawless Ruby, reload your save and repeat until you get one.
Killing dragons is one way of obtaining this rare gem.

A fixed Flawless Ruby can be picked up from Shriekwind Bastion. Halfway through the dungeon, a single throne can be found in a blood-spattered room. Examine the floor near the unlit fire pit on the right for the ruby (ruby is not always flawless).

If you have Prowler's Profit, you'll be drowning in flawless gems of every type.

Once you have gathered the necessary ingredients, return to the shrine and inhale the fumes to speak with Peryite. He will give you a task to kill his current overseer Orchendor that has forgot about his duty. He will also state that you will replace Orchendor as his new champion. Orchendor is a mage (fire/ice) who resides in Dwemer underground fortress called Bthardamz. It will be quite a challenge to find him there, because Bthardamz consists of a few levels and areas filled with Afflicted (minions of Peryite that Orchendor guides) and Dwemer Automatons. Try to follow the quest marker and check the local map often so you can keep an eye of your destination and eventually you will find your target. Assassinate Orchendor, gather his valuables and loot and be gone using the special dwemer lift. When you return to Peryite he will reward you with the Spellbreaker. A detailed walkthough is available here

Bugs

After you give all the acquired items to Kesh, you can pick his pocket and get all the items back before he created the fumes. This doesn't interrupt the quest because he is still able to create the fumes (even without those ingredients), and the Dragonborn is still able to proceed with the quest.

Room before encounter with Bthardamz may have a bugged floor, in which the player falls to the door leading into the lower parts of the dungeon. (Confirmed on 360) Try using whirlwind shout to get to the other side.

(Xbox 360) with the above bug with the missing floor, any follower will fall through and be lost.

(Confirmed on PS3 and XBOX) If the player talks to Kesh while he is creating the fumes, the quest will not be updated.This may cause the trophy/achievement "Oblivion Walker" unobtainable,since the player cannot obtain Spellbeaker.
(PS3) Perite have not give to me the Spellbreaker, I hat talk to him after kill Orchendor, and he just talk some boring things and then! Give me nothing (that scrimpy skeaver)... .
(PC) There seems to be a balance bug with the afflicted, their acid spray kills the Dragonborn in one or two hits at level 18, making abuse of traps the only way to fight them. (in the active effects tab, it says "20 poison, 4 seconds {80 damage} which is insane because they can all do it as much as they feel like.
(Xbox 360) I had the same problem. I completed the quest but did not get Spellbreaker. I tried reloading and doing it again but no luck. [Try this: The Spellbreaker is a shield. Even though it didn't say it was added to my inventory, it was there once I finished the quest. So check there first as you might think you don't have it when really you do.]

Update: You can fix this by killing Kesh, taking the ingredients, and activating the cauldron yourself.