Upon triggering, a perfectly spherical boulder of enormous size either falls off a pedestal, or comes out from a hidden panel, and rolls towards the characters. They may attempt to get out of the way, using Dex, AC, or Saving throws (GM's choice), or they may try to outrun it (50-50% chance of success). Certain magics may help. Anyone hit by the boulder takes 6d6 of crushing damage!
Optional: There's an inclined plane at the other end of the area, which, once past the characters, the boulder rolls partially up and then comes right back down again, rolling back at them.
Optional: There are inclined planes on BOTH ends of the area, and the boulder keeps rolling back and forth for an extended period of time!