Talk to Aldous in the Duke's Audience Chamber, and choose to Recover stolen item. Aldous tells you that an item was stolen from the castle's stores a few days ago. It was a possession of the Duke's, and a treasure of the realm: the Wyrmking's Ring. The band was stolen under heavy watch, and Aldous suspects it was an inside job, someone from within the castle. But first, the ring must be reclaimed.

Accept, and this will begin the quest:

The Wyrmking;s Ring
Learn More of the Thief.

Objective
Royal Orders Quest
A thief made off with the duke's magick ring, a treasure of the realm. It must be reclaimed

Details
Speak with castle residents to discover what you may of the thief.

Aldous suggests you ask around the castle. That ring is a treasure to all Gransys, and a symbol of the duke's authority. It must be found.

Follow the red quest markers. Talk to Pering first. He is a servant and can be found around the castle, sometimes he is in the audience chamber itself (near Aldous) and sometimes in the northwest corner of the building.  Pering tells you that even should the thieves escape sight, the castle's construction hinders all who do not know it well. He suggests the villain either had a guide, or had time to grow familiar with the castle.

Next, speak to Ser Gyles. Ser Gyles can be found on the second floor, usually in the Storehouse. He reports hearing of an unfamiliar man, who had the looks of a sorcerer, seen within the castle's halls.

Now speak to Ambrose, who should be standing either in the audience chamber or on the 2nd floor to the west. You can recognize him by the big scrolls he is holding. Ambrose says that if there's talk of a sorcerer, it must be Salomet.

Talk to Aldous and report your findings. He tells you that the name "Salomet" holds bitter memories. There was a time when the Duke held a council of magick scholars here in Gran Soren, tasked with ensorceling grimoires and trinkets. Salomet sat at the head of the council. His genius was unmatched, save by his ambitions. Once their research bore fruit, Salomet betrayed His Grace and fled with the findings. He disappeared, leaving the corpses of his fellow scholars in his wake.

You now need to inquire about Salomet. Talk to Mellard, who should be located either in the Knight's Manor, or near the Cathedral, or in the Gran Soren Noble Quarter. You learn that Salomet is an infamous archmage, and that he has been seen in the ancient quarry. You are now tasked with confronting Salomet.

The Ancient Quarry is to the west of Gran Soren. Travel there, and enter through the Ancient Quarry North Entrance. Go down the first hallway, soon there will be a cutscene, where you confront Salomet and his men. There is a battle. Use your Area of Effect attacks, as the enemies will be grouped together at first. Eventually, when they are about to lose the fight, Salomet concedes and escapes the battle. You now need to return to Gran Soren to report Salomet's escape to Aldous. Head back to the Duke's Demesne and speak with Aldous in the Audience Chamber. He wants you to continue pursuing Salomet.

You now have to renew your inquiry into Salomet. Go to the fountain square in the Urban Quarter, and talk to Brice who is standing behind one of the Stalls. He suggests you search for Salomet in the Bluemoon Tower. To get there, trek northeast past the Windbluff tower, and then take the narrow mountain path to the east.

Salomet can be found in the northern area of the tower, you will have to circumvent the center area and use the southern passages to get there, climbing up the many flights of stairs along the way. You will find Salomet in the Tower Summit. The Wyrmking ring lets him cast his spells faster. He teleports all over, and summons a lot of minions such as Skeletal Knights and Skeletal Lords to do the fighting for him. Eventually he gets worn out from summoning, at which point you need to focus all firepower on him.

Defeat Salomet. You get:

Wyrmking's Ring
A band that exudes mystic power. Merely possessing it allows for quicker incanting of spells.

Note:
You may want to keep the Wyrmking's Ring, as it increases casting speed, thus considered pretty powerful. Take it to Mountebank, in the Black Cat, and have him make a forgery of it. It will cost you a lot of gold (like 100,000G) but it is worth it. Hand over the fake Wyrmking Ring to Aldous. He will not suspect that it is forged. The only drawback is that you might miss out on a good cloak, the Paladin's Mantle. However, choosing between the Paladin's Mantle or Wyrmking's Ring, I believe that the Wyrmking's Ring is better than the Paladin's Mantle.

Then, ferry back the ring and hand it over to Aldous. This completes the quest, The Wyrmking's Ring.