As you round a corner, you are confronted by an army of thousands of tiny glass figurines. Each tiny little soldier is armed with a tiny little obsidian weapon. Most are infantry. Others are archers. Some are cavalry. A lucky few are riding glass elephants. There is also artillery. When you turn around, your retreat is also blocked by thousands of figurines. A tiny general waves his tiny sword, and they attack.
The army of glass will attack until they have received 200 hit points worth of damage. Area attacks like fireball do quadruple damage. Give the party some leeway in terms of allowing mass destruction through clever actions, such as by summoning large creatures to crush the glass, using the environment, etc.
Their greatest threat to the party is the artillery, which fires stored kinetic energy marbles. These are +6 to hit and deal 3d6 damage. Every party member is subject to one of these attacks each round until the army flees.