Batrachian Metamorphosis

You force one creature you can see within range to make a Constitution saving throw. On a failed save, the creature's flesh begins to roll in upon itself as the creature's body slowly turns itself inside out, gaining vulnerability to all damage and disadvantage on all attack rolls and saving throws until the effect ends.

The affected creature makes a Charisma saving throw at the end of each of its turns, ending the effect after three successes, or transforming into a Gibbering Mouther under the DM's control after three failures cured by a Wish spell or similar magic.