This is a +3 weapon that deals 1d6 bludgeoning damage with 10-foot reach, also deals an additional 1d8 radiant damage.

When you hit a fiend or an undead with this magic weapon, that creature takes an extra 2d8 radiant damage; you can cause the mace head to explode with holy flare on the targeted creature you hit, dealing the extra radiant damage to other creatures within 5 feet of it as well. If the target has 30 hit points or fewer after taking this damage, it must succeed on a WIS saving throw or be destroyed. On a successful save, the creature becomes frightened of you until the end of your next turn.

If you don't have a spellcasting DC save, your DC save = 8 + your proficiency bonus + your WIS mod.