Warhammer: End Times - Vermintide

To unlock the secret cellar in the house in the middle of this mission you have to destroy 3 glowing stones. The first stone is shortly before you are reaching the house on the left side in the swamp "fog" section. The second stone is close to the shore near the house and the third stone is in back of this house on a wall. Destroy all three stones and the cellar in the house opens, pick up the grimoire and the achievement should be unlocked.