The Master Below

 is a side quest in.

This is the largest side quest of the game, and awards the most experience. Completing it requires navigating every level of the Endless Paths of Od Nua. This in turn triggers other side quests.

While access to this area is available at the end of Act 1, the battles in lower levels require a party strength that is difficult to acquire before Act 3.

Synopsis
The Watcher's new steward has informed them that, long ago, a powerful Engwithan constructed a labyrinthine dungeon underneath Caed Nua now known as the Endless Paths of Od Nua. Od Nua was killed by the people of his time for his deeds, but the steward claims that a powerful force still dwells within the ruins. Whenever someone attempts to take possession of Caed Nua, this 'Master Below' stirs awake and sends forth a flood of monsters to reclaim it. If the Watcher does not deal with this sinister presence, they may eventually share the same fate as the Aedyrans who tried to claim this land long ago.

Walkthrough
See the pages for the Endless Paths of Od Nua. The quest progresses at each of the following levels: Killing the Adra Dragon Sefyra in Level 15 ends this quest. If instead you agree to help, the adra dragon will give you The Dragon's Amulet, and the quest continues:
 * Level 1
 * Level 5
 * Level 8
 * Level 14
 * Level 15
 * Return to Level 1 and place amulet on the giant's hand (in the northwestern corner), transferring the dragon's soul into the amulet. This will advance the quest and earn you a fair amount of experience.
 * Find Falanroed at the shrine to Galawain within the Black Meadow.
 * If you tell her what's happening, the amulet breaks, and you have made a powerful enemy of the Adra dragon. To complete the quest, you will have to return and kill the Adra dragon. To help with this, Falanroed teaches you Scale-Breaker.
 * If you offer the amulet as a gift, or just put it on her, she will detect the trap, and you will have to fight her. When you have defeated her, the Adra dragon will take over her body. You have made a powerful ally. She will tell you that Thaos is in Sun in Shadow, and that you can defeat him. She then leaves.
 * Kill Falanroed to get the unique armor Autumn Fire and the unique pollaxe Half-Mast, and then kill the adra dragon, who drops an Adra Dragon Scale.
 * Completing the quest grants a massive amount of experience. You can now loot the dragon's hoard at the east edge of her lair in Endless Paths of Od Nua Level 15. After completing the quest, speak with the Steward to gain a bonus to Prestige (+8 ) and Security (+8 ).