Celestial hammer

You befall a celestial hammer on a target between range. Make a spell attack. On a hit the target must make a Con saving throw. On a fail the target takes 1d8 bludgeoning damage and 1d8 radiant damage. The target then, stunned by the light, has a speed decreased by 10ft until your next turn. On a success the target takes only the bludgeoning damage and is not "stunned".

Cast in higher levels:  For higher levels the damage increases by 1d4 for both bludgeoning and radiant. 

(Note that the target is never actually under the stunned condition)