Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
This very large dam has a small ledge that runs the length of it.
When a target reaches the middle, the ledge retracts. All targets must make a very hard Dexterity saving throw to not fall. Those that do not fall must make a very hard Strength (Athletics) check each round to move or fall.
When a target falls, they take lethal bludgeoning and slashing damage, as the wall is slightly curved outwards, causing the targets to slide down a rough concrete wall for hundreds of feet.