Rod of Entropy

This magic weapon is a short, blackened iron shaft topped by three skulls sculpted to look as though they have melted together. You gain a +2 bonus to your attack and damage rolls made with this magic weapon. As a Magic Action, you can create a 15-foot Emanation of a black aura. Each creature within the Emanation must make a DC 17 Constitution saving throw, taking 3d6 necrotic damage on a failed save, or half as much damage on a successful one. A creature must make the Constitution saving throw whenever the Emanation enters the creature's space, or whenever a creature enters the Emanation's space for the first time on a turn or starts its turn there. Once you use this property, you can't use it again until you finish a Long Rest.

Proficiency: simple, mace Source: Treasures from Oerth p. 24 (Homebrew)