Corpse Slayer Warhammer

You gain a +1 bonus to attack and damage rolls made with this magic weapon. When you hit an undead creature with an attack using this weapon, the attack deals an extra 1d8 damage of the weapon's type, and the creature has disadvantage on saving throws against effects that turn undead until the start of your next turn.

Versatile: This weapon can be used with one or two hands. A damage value in parentheses appears with the property—the damage when the weapon is used with two hands to make a melee attack.

Proficiency: martial, warhammer Source: Explorer's Guide to Wildemount p. 266 (Third Party), Player's Handbook (2014) p. 149