Warning! You’ll want to begin this mission before you complete the second-to-last main mission ‘Touching the Sky’, otherwise, Isran will rally his troops together and start the final main mission ‘Kindred Judgement’, making it near impossible to start this side-mission.
Have a chat with Sorine and she’ll mention someone called Florentius Banius who’s worth recruiting, but you’ll need to speak with Isran about it first (hence our warning above). You’ll need to head to a place called ‘Ruunvald Excavation’ (found south-east of Tovald’s cave).
Climb the mountain until you reach the snowy entrance to the cave. You can read the journal in the tent (should you want some back-story), otherwise, head inside the double wooden doors.
Run down the stairs, kill the Charmed Vigilant and loot his body for a decent amount of loot and potions. Downstairs is another charmed vigilant and an apprentice locked chest. Head into the main cave system and kill any vigilants that get in your way.