Nightwrought Armor

Armor (medium or heavy), very rare (requires attunement)

This armor is created from nightore, a special substance found only in the Shadowfell. While wearing this armor you have resistance to necrotic damage. In addition, when a creature within 5 feet of you hits you with a melee attack, you can use your reaction to inflict 2d8 necrotic damage on the target, as your armor sucks the life essence from it. Undead are immune to this effect.