Online:The Corrupted Stone

Quick Walkthrough
# Talk to Erold at the Crossroads Tavern.
#;Jewel of the Skies — Search the Dwarven ruins for the Skystone Amulet.
# Enter Mtharnaz.
# Search for the Skystone Amulet.
#;The Three Charges — Empower the Skystone Amulet at the Mundus Stones.
# Visit the Lord, the Lady, and the Steed Stones in Western Craglorn.
# Recharge the Skystone Amulet.
#;Cleansing the Stone — Use the Skystone Amulet to cleanse the Warrior's Apex Stone.
# Gain access to the inner portion of the Ruins of Kardala.
# Defeat Izrunath the Corruptor.
# Cleanse the Apex Stone.
# Return to Erold in Belkarth.

Detailed Walkthrough
Erold, one of the Star-Gazers, thinks he has a way to discover what the Celestials really are. He will use Apex stones. He even knows where one of the great mundus stones is. It is locked within the ruins of Kardala. The only way to open the ruins is with a Skystone amulet. 

Erold thinks Mtharnaz, a Dwemer site, is the mostly likely place to find a Skystone amulet. 
Jewel of the Skies
:New Task: Jewel of the Skies — Search the Dwarven ruins for the Skystone Amulet.
Find the amulet inside the ruins. It is in the final room with the Brass Hatchling, on a desk in the water. When you take it, The Celestial Thief appears and talks to you:

''"Listen closely. I can't appear for long or the Enemy will see. Erold means well, but he can't accomplish the task ahead. You must do this. The relic you hold is called the Skystone Amulet. It will grant you access to the Warrior's Apex Stone—but it is capable of so much more. Go to the Mundus stones of the Lord, the Lady, and the Steed. These are the Warrior's charges, whom he has sworn to protect. The Skystone Amulet was designed to capture the aetherial energy of the Mundus stones. You will need that energy for the task ahead. I'm sorry I can't tell you more. I've already stayed too long."

:Task Completed — You uncovered the Skystone Amulet from within Mtharnaz.
The Three Charges
:New Task: The Three Charges — Empower the Skystone Amulet at the Mundus Stones.
You need to power the amulet at the Lord, Lady and Steed Mundus stones located in the western half of Craglorn. The Thief will continue her tale as you reach each one:

"Good. You've made it to the first stone and the amulet is working. I don't have much time, so listen carefully. The Serpent betrayed us. He poisoned the Warrior's mind and turned him against us. But my hope is that the aetherial energy from the Mundus stones will allow us to weaken the Serpent's grip on the Warrior."

"You have the aetherial power of another of the Warrior's charges. Good. The Serpent's followers, the Scaled Court, range all across Craglorn. They found the Warrior's Apex Stone. They weakened the stone with foul magics. I believe that the aetherial power of the Lord, the Lady, and the Steed can restore the Warrior's Apex Stone."

"At last, the amulet contains the aetherial energy of all three of the Warrior's charges. Now, hurry. The power of the Lord, the Lady, and the Steed can strengthen the Warrior's Apex Stone and weaken the Serpent's hold. Go to the ruins of Kardala, at the center point from where these stones rest. The Apex Stone lies within." 

:Task Completed — You recharged the amulet with Aetherial magic.

Now that you have the amulet you can open the Throne of the Warlord inside the Ruins of Kardala. You need to step on the stones of the Lord, the Lady, and the Steed, which should light up, allowing your group to enter. (Note that you no longer need three players to stand on the stones at the same time; one player can stand on each stone in succession.)

Cleansing the Stone
:New Task: Cleansing the Stone''' — Use the Skystone Amulet to cleanse the Warrior's Apex Stone.
As you enter, you hear the Thief's voice once more: ''"At last, the Warrior's Apex Stone stands before us. I sense the Serpent's power in this place. I must flee before I am ensnared as well. The Skystone Amulet is charged with the aetherial power of the Lord, the Lady, and the Steed. Use it to cleanse the Apex Stone of the Serpent's corruption."''

You then need to fight and defeat Izrunath the Corruptor. He is initially supported by two Scaled Court bruisers, and summons two giant snakes when his health drops to below half. After he is defeated, you can cleanse the Apex Stone.

Aspects of the Lord, the Lady and the Steed appear to weaken the Serpent's hold on the Warrior.
:The Lord: "Great Warrior, I am your strength. Forsake this madness and return to health!"
:The Lady: "Great Warrior, I am your mercy. Forsake this madness and remember peace!"
:The Steed: "Great Warrior, I am your swiftness. Forsake this madness and ride against our enemy!

:Task Completed — You restored the Apex Stone with the magic of Aetherius.

Once the stone is cleansed, a projection of the Serpent appears and addresses you: ''"You have purged my venom from the Apex Stone. Clever. Too clever for a mortal. She helped you, didn't she? I can smell her fear upon you. No matter. I will find her and consume her in time, just as I have her fellow guardians. Do not think you have succeeded here. The damage remains. The Warrior has fallen from the sky and is trapped in mortal form. He is mine."

After the projection disappears, you are free to return to Erold at the Crossroads Tavern in Belkarth.

:"You're back! I was beginning to worry for your safety. Did you find the amulet? Tell me everything that happened!"
:The Serpent poisoned the Warrior's Apex Stone, but I restored it.
:"Restored the Apex Stone? Incredible! Then my predictions were more accurate than I expected and you've done more than I could've asked. I'll keep the amulet safe here among the Star-Gazers for now. There may be more we can learn from it."''

Notes
This quest is, in fact, no longer absolutely obligatory, although desirable for zone completion.
It is possible to sneak past Izrunath the Corruptor and cleanse the Apex Stone without defeating him first.
 

Quest Stages