Overview
Berengar’s Blade is a secondary quest in The Witcher 3: Wild Hunt, set at Kaer Morhen and in the valley around it. It fails if not completed before the council in The Battle of Kaer Morhen. Talk to Vesemir before you travel to the Isle of Mists, although finding the diagram without his help also completes it.
Berengar’s notes
In a chest in a ruined, cell-like room by the western wall of the keep, west of the grindstone in the middle courtyard, Geralt finds notes left by Berengar, a witcher of the Wolf School. Geralt decides to ask Vesemir.
Vesemir tells how Berengar once tried to forge a special sword but lost the diagram in a fight with a chort he had to flee from. He points Geralt to the cave where it happened and advises going along the river.
The cave
Deep in the cave the chort is still there. After killing it, search the cave for a bag containing the Tor’haerne diagram. Behind some stalagmites in the same cave lies the Superior Wolven boots diagram, so grab it while you are there.