Werebane

Silver Dagger - Werebane is an dagger+1, but when fighting lycanthropes, it acts like dagger+4, having a higher change to hit and do more damage. It is found at Lycanthrope Island of the Tales of the Sword Coast expansion pack.

In-game description
"Silver dagger - Werebane: This +1 dagger acts like a +4 dagger vs. lycanthropes. Dynata Goldenhand, an elven ranged who wished to ridher homeland of its were-jackal problem, forged the dagger. She was killed by a tribe of orcs in the Greypeak Mountains as she tracked the jackalwere leader."