This is a single room dungeon for “the shattered key” quest within the “for the king” campaign. The cellar location is randomized but was on “the rogue isles” for me. Only a single party member can enter and do the battle so bring your strongest and have some healing herbs. My battle was against a gelatinous cube at level 6 and was pretty easy as I was level 9 by the time I found the location.