Wolfbane

This older silvered short sword remains in good condition and is inscribed with "Wolfbane" written in runes along the blade. The weapon is surprisingly light and appears to have a mithril core and while unassuming, it clearly once held some significance. The weapon deals +1d4 damage to all shapeshifters (not just werecreatures), and prevents werecreatures from regenerating during a turn that follows them being struck by this weapon.
The downside is that, the first time the wielder of this sword sees the moon, within a reasonable timespan, they must make a Wisdom Savingthrow vs DC 8 or choose between either fleeing indoors or go berserk and attack friends and foes alike. When wielding the sword against werecreatures or while knowingly going out into the night to potentially fight such, the wielder may make the savingthrow with advantage.