I like Crothians idea - Capture them, let them go and then scry on them and find out what they do!
Another possible:
Have the players seen the undead army yet? If not, then it should be staging outside the ruins, or at least not in the section the players are in.
Anyway, let the players see them. If they're like most players I know, then they'll dither about what to do for a while. If they come to a decision, or you get bored, then have the Mummy start to move it's army out. As they start leaving, he casts a 'plot device spell' on the ruins.
The complex reforms, seals and sinks into the ground. With the players still inside! The death traps switch on. The hidden guardians come out to play. There is an 'escape capsule', if they can get to it then they live. This could be a teleporter to somewhere interesting, an appratus of Kwalish that digs through the ground or a stargate. Whatever sounds fun really?
By the way: mummys, Shifter-Swarm sand things and ancient ruins. That sounds really cool.
