Way to unlock the quest: Talk to Cassandra in Skyhold after you complete the Here Lies the Abyss main quest.
Quest giver: Cassandra (M12,3)
You receive this quest from Cassandra (M12,3) in Skyhold. First, you need to complete the "Locate the missing Seekers", and return to Caer Oswin. Upon your arrival (M25,1a) head towards the castle and enter through the wooden door.
Inside, you encounter several opponents. Loot the corpses thoroughly, because there is a key on one of them (M25,1b). Then, follow the corridor to the yard.
Defeat the rest of the opponents on the yard and find another key (M25,1c). Climb up the wide stairs to the big door. Past the door, you find several opponents more, this time shielded ones, as well. Climb up the stairs again, and find the dying Seeker (M25,1d), which activates a cutscene.
After it ends, climb upstairs and climb towards the next door. Behind this one, there is the Lord Seeker Lucius and his company. After the conversation, you will have to deal with him. He is not too much of a problem, nor are his minions. After the battle, search the corpse, thanks to which you find a legendary weapon and leave the location towards Skyhold. Now, see Cassandra again and talk to her to complete the quest.
Rewards for completing the quest:
- Experience (lots)
- 400 Influence points
- 3 Power points