Long Fang of the Moon

You gain a +3 bonus to attack and damage rolls made with this silvered longsword. Weapon attacks using _long fang of the moon_ deal an extra 2d6 radiant damage against creatures. When you hit a shapeshifter with this weapon, it makes a DC 30 Constitution _saving throw_ or reverts to its original form. If it fails this saving throw, you may choose to prevent the shapeshifter from transforming for up to 10 minutes.

Source: Level Up Advanced 5e · OGL-1.0a · 5.1