Starcraft Plasma Charged Melee Weapon Template (5e Equipment)
From D&D Wiki
Weapon gets 1 type of additional damage equal to weapon's normal damage multiplied by 2 (e.g. if weapon's normal damage is 2d6, additional damage will be 4d6). You can choose fire, acid, lightning, or necrotic damage for additional damage type. Each weapon can only deal 1 type of additional damage, and can never change unless it is completely reforged.