In response to a creature vocalizing, you make a sharp sound. and hold a finger up in an attempt to interrupt them. The target must succeed on a constitution saving throw or become silenced as its mouth is fused shut for the duration. If you silence a target mid spell-casting, the targets spell slot is not expended but its action is wasted. 

As a bonus action. on each of its turns, the target may attempt a strength saving throw against your spell save DC as it tries to tear open its mouth. On a success the target takes 1d8 slashing damage and is no longer silenced. Alternatively, as an action, the target can use a bladed implement to cut open its mouth, taking 1d8 slashing damage and ending the silence. 

At higher levels. When you cast this spell using a spell slot of the 4th level or higher, the damage increase by 1d8 for each slot level above the 3rd.