Cramp

You cause one muscle on an opponent to cramp up. Pick a finger, thumb, foot, calf, eyelid, etc.  On a failed save the target takes 1D4 damage and their muscle contracts until a successful CON save is made. On a successful save, the target takes 1D4 damage but no other effects.

Examples

A finger cramping would give disadvantage on attacks made with a bow.

A thumb would give disadvantage on holding a weapon, or a rope if climbing

A foot would give disadvantage on most movement

A calf would give disadvantage on speed, speed would be reduced by 10'

An eyelid would give disadvantage on aim

An arm would give disadvantage on a grapple