Speak No Evil

Wondrous item, uncommon
  This headgear – a gag consisting of two leaden ‘hands’ connected by thick leather straps – is designed to make speech impossible.
  Applying the Gag. As an action, you can attempt to gag a creature you are grappling. Make a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics). On a success, you get the gag in place and the leather straps magically fasten. On a failure, you do not gag the target.
  Removing the Gag. A creature can use its action to make a DC 16 Strength check, removing the gag on a success. The knock spell can also unfasten the leather straps, causing the gag to fall off.
  The Gagged. A creature who is gagged using Speak No Evil can’t speak or cast spells that require a verbal component. In addition, should they attempt to cast a spell, the gag heats up, dealing 1d4 fire damage, plus 1d4 additional fire damage per level of the spell (cantrips deal 1d4 fire damage total). This damage occurs directly after the spell is cast, prompting a check to maintain concentration.