Antiplanar Weapon

Antiplanar arms are weapons created specifically to hunt down creatures touched by extraplanar powers. Crafted by welding together mortal silver and infernal iron, and bound with heavenly cloth, this weapon's dark silver and gold appearance draws the ire of celestial and fiend alike.

When a creature that you can see within 180 ft. of you deals radiant, necrotic, force, or psychic damage, you may use your reaction to point this weapon at the creature, drawing it if it is currently sheathed. If you do, mark that creature until the next dawn. When you attack a marked creature with this weapon, roll this weapon's damage dice twice and use either roll when making an attack. Additionally, your first attack each turn made with this weapon against a marked creature has advantage. You can only have one creature marked with this weapon at a time, and you cannot mark another creature with this weapon while it always has a creature marked. 

 

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Versatile, Sap