The charm will not take damage when right clicking on a demon but will take 5 damage for every trade made (less with unbreaking enchantment). The charm has 50 durability and will break after the 11th trade. Trading with a demon using the Devils Tongue Charm does not lock the trade, until the charm breaks. Stopping after 11 trades and switching to a new Devils Tongue Charm will allow players to trade indefinitely with a demon. There is currently a bug where when in the trading screen the charm will not appear to take damage.