The Middle Chamber is located in A Small Pyramid, which becomes unlocked during the Quest for the Holy MacGuffin.
A primary stat of 165 is suggested. If your main stat isn't high enough, you'll see the following message:
(It is recommended that you have at least 165 <mainstat> to adventure here.)
- Are you sure you're walking enough like an Egyptian (or at least a bad-ass adventurer) to go in here? Or will it send you running back home to mummy?