Dawnguard Faction --Bolstering the Ranks

Dawnguard Faction --Bolstering the Ranks

When you talk to Sorine Jurard she tells you about the great concern they feel over the situation and the fact that Isran is very worried.  Him being worried worries them, so she asks you to ask Isran about recruiting Florentius Banius.  When you do so Isran is reluctant at first, but eventually agrees, and tells you that Florentius lives in Ruunvald, which is an excavation site located in the mountains east of Shor's Stone and north of Riften.

The Rescue and Recruitment of Florentius

When you arrive at the site it is obvious that something is wrong.  You should read Volk's Journal in the tent outside...  The Vigilant have been enthralled and they attack you pretty much on-sight.  Battle your way through the caves until you reach the end where you find Florentius locked in a cage and under attack!  Kill off the enemy and then talk to him to recruit him -- he is something of a nutter he is.

Interestingly enough once you recruit him that completes the quest BOLSTERING THE RANKS so you have nothing to do but head back to the fort!