To start this quest you should talk to Paladin Warden.
We discovered that some prisoners had vanished from the cells beneath the Magister Barracks in Arx.
The Missing Prisoners
Related points of interest
Quest stages of The Missing Prisoners
1. Prisoner Were Used For Source
Find and read Isbeil's Notes.
We learned that Isbeil had seized the prisoners in order to carry out experiments on them.
2. Found Dead Prisoners
Enter the Isbeil's Laboratory.
We found some dead prisoners in Isbeil's laboratory.
3. Quest Reward
Report back to Paladin Warden.
4. Knows Magisters Were Investigating
We learned that the Magisters had been investigating the disappearances of several prisoners from their cells beneath the Magister Barracks, prior to the Paladin uprising.
5. Close Left ARX
We have ventured onwards without discovering what happened to the prisoners.