Evocation cantrip (arcane)
Casting time: 1 action
Range: 30 feet
Components: V, S
Duration: Instantaneous
Your target must succeed on a Strength saving throw or you magically undo the fastenings of their pants which then drop to their ankles. Mundane belts, suspenders, or any other device does not prevent however the simplest of magical versions will.
They can use an action to return their pants to normal. With the pants around their ankles they are considered to be on difficult terrain. If they move more than 5 feet they must succeed on a Dexterity saving throw (same DC) or fall prone.