Eventually one of the players will notice a greenish glow near the front of the house. Checking on this reveals a shapeless glow that seems to be forming in the front room. Almost no spells or weapons will harm it. 90% magic resistance (SR 30) and +3 or better weapons to harm (DR 25/+3). This is a partial from of the Arch-magi Xoth, he is attempting to reform a solid body with his dormant powers spread out through the collected masks. The fact that so many of the masks are here together, and Charilus is attempting to manipulate their powers is giving Xoth slight access to this plane of existence.
Fortunately he will be unable to from a physical body but he will come fairly close and will attempt to seize the masks. The glowing form will eventually coalesce into a man sized blob, it then moves toward the location of the nearest mask. It is not solid enough to be blocked by the players. If they do attempt to stop it, Xoth will give a small demonstration of his power. The figure will make a waving gesture (mass telekinesis) and all party members in the room must make a save (DC 25) (2E versus spells) or be tossed through the air as if from a strength score of 20. This will result in 2d6 points of damage to all that were thrown. It will stand about confidently as the party and Charilus attempt to cast spells upon it or strike it with weapons.
Eventually Charilus or a party member must suggest a retreat since Xoth is nearly impossible to hurt in this form. As they flee into the night Xoth will follow, when they reach the street he will summon up 2 gibbering mouthers 3E MM 104. These nightmarish creatures come from a wholly unknown plane of existence so the players will be totally unfamiliar with them. The gibbering mouthers will attack the party immediately, fighting until slain then dissolving into greasy black smoke.
Xoth will gesture impatiently for the masks. Charilus will make a hasty decision and tell the party to “Take the masks and run! Run as fast as possible and find that tomb!” He will then take his staff of power out and prepare to break it in a retributive strike. Even Xoth will be shocked at this action and attempt to flee. The last sight the players will have is a powerful magical explosion that devours both Charilus and the glowing figure.
Note: Neither was actually destroyed here- Xoth was merely forced back off this plane and Charilus has a contingency spell to save his life but he will choose to “play dead” for a while.