The Horn of Ehlonna

The wielder of this dagger has +1 to attack and
damage rolls with this weapon and advantage on
saving throws against poison, necrotic and disease
effects.
If this dagger deals damage to an undead, fiend or
demon then the owner may deal an extra 1d6
radiant damage.
All that remains of the Unicorn Ehlonna to aid
others in her fight against the evils of the world