When the Main Chamber is finally free of Hobbe forces, poke around and open the two chests nearby to earn yourself a total of 1,000 Gold and a handy-dandy Flame Augmentation. If you’ve been wondering what Grandma’s Hexagon Key is for, now’s the time to get your answer. Approach the door to the west and use it on the strange-shaped lock. Bit of an anti-climax, really.