Virion's Viscous Diarrhea

 creature of your choice that you can see within range suddenly experiences uncontrollable, explosive bowel movements. The target must succeed on a Constitution saving throw or take 4d6 necrotic damage and be incapacitated for the duration of the spell, due to the painful, involuntary excretions.

On a successful save, they take half damage and are not incapacitated. On a failed save, the creature is incapable of taking any action on their turn other than a constitution saving throw to break the spell. A failed constitution save on their succeeding turns will result in them taking a further 3d4 + 3 damage.

This spell has no effect on those immune to necrotic damage, and on constructs, because constructs don't have the capacity to defecate.