Upon triggering, all exits seal up, holes in the wall open, and water rushes in, quickly flooding the area. The water only comes up to about mid-chest height on a human, then freezes solid in an instant. Characters are trapped, and possibly freezing to death.
The ice is kept frozen magically, but can be melted or broken by normal means. There is a 50% chance the characters have their arms above or below the surface of the water when it flash-freezes (roll for each arm of each character). If the players did not specifically state their characters had a weapon or other implement in hand before the trap is activated, then they do not. Great strength may or may not allow a character to shatter the ice, freeing their arms or equipment (GM's decision).
Any character who is not caught in the initial flooding because they were hovering in the air, hanging off the wall, etc., is free to move around on top of the ice once it freezes. There is a hidden magical control for the trap, secreted somewhere in the area, which will unfreeze the water, open holes to let it drain out, and then unseal the exits.
Optional: As soon as the water freezes, a hidden door opens, and monsters or enemies pour out!