Bolstering the Ranks

Background
''It's been suggested that the Dawnguard could use the help of a man named Florentius, a former Priest of Arkay. I need to speak with Isran to find out if he knows where Florentius might be.''

Walkthrough
Speak with Sorine Jurard after returning to Fort Dawnguard with Dexion Evicus. She tells the Dragonborn to ask Isran about recruiting Florentius Banius. Isran is reluctant at first, but relents, saying Florentius lives in Ruunvald, located in the mountains east of Shor's Stone and north of Riften.

Ruunvald Excavation
Upon locating Ruunvald Excavation, Volk's Journal can be found outside in a tent hinting at something amiss with the ruins. All of the Vigilants that had been working on the ruins have been subjugated and will attack on sight. Head inside and down the wooden ramp, past the crate with Discovering Ruunvald, Vol I atop, and defeat the two Vigilants. Go north into the tunnels and past the crates and a cart with three pieces of malachite ore. Kill two more Vigilants and head on past a tripwire that sets off an oil trap. There are two chests in the tunnel.

The tunnel ends, opening up into a cavern with two more Vigilants and a husky. Some leveled potions can be found on the western wall and a malachite ore vein is under the ramp. Discovering Ruunvald, Vol II is on a crate next to a bed roll by a large stone pillar.

Go into the next tunnel, head east and down the wooden ramp past another chest. Defeat another Vigilant and husky and head onwards through the tunnel to another cavern with a ramp leading deeper into the cave. Another Vigilant is posted here and Discovering Ruunvald, Vol III is on a table. Go through the tunnel and past the pressure plates that are linked to crossbows. The tunnel opens up into another large cavern that is the main entrance to the ancient ruin itself.

Volk, four Vigilants and a husky wait here. Cross the bridge and head down the wooden ramps to the floor level. A small sleeping area with a chest is by the eastern wall. To the south is an archway that leads into the Ruunvald Temple.

Ruunvald Temple
Upon entering the tunnels, head straight to the candlelit area where the book Minorne is. Go south to the large cavern where Florentius is being held in a cage while being attacked by Minorne, Moric Sidrey, and a Vigilant.

Killing Minorne results in the death of all remaining charmed vigilants. Check Minorne for the key to Florentius' cell. After a brief "discussion" with Arkay, Florentius will agree to assist the Dawnguard and will head to Fort Dawnguard. This completes the quest.

Trivia

 * If the Dragonborn assumes the Beast Form, Florentius will be hostile but cannot be killed. If the Dragonborn then runs away, exits the cave and then ends Beast Form, Florentius will no longer be hostile when next met.

Bugs

 * This quest must be started before completing Touching the Sky .  Otherwise, any attempt to speak with Isran will cause him to call the Dawnguard together and trigger the start of Kindred Judgment, making it impossible to ask him about Florentius.