Glass Dome

A titanic glass dome covers a city. It's unbreakable by modern might or magic, and bears only a few small cracks where some terrible prehistoric force was set against it. The city was constructed under it, in the shadow of these ancient ruins and wonders. After a climactic fight scene, as the PCs are running from (or about to be running from) an agent of Evil and Chaos, they find call to be up on the dome. The black-armored death knight stomps onto the glass dome, and the dome rings out like a monumental bell or gong. The glass warps/melts/cracks underneath the death-knights boots.