Vanesax's Ancient Dragonbolt

A bolt of blue lightning strikes a non-magical weapon within 15 feet of you, coating it in a freezing white energy. Until the spell ends, that weapon becomes a magic weapon with a +2 bonus to attack rolls and damage, its damage type also changes to Lightning or Cold (chosen by the caster). All weapon attacks made with a weapon affected by this spell apply 1 stack of Frostbite to their target. Upon reaching an amount of Frostbite equal to the targets Constitution modifier + 2 (a minimum of 1), the afflicted target becomes vulnerable to Lightning and Cold damage for 15 minutes. A target can reset their Frostbite stacks by coming in contact with fire or taking a Short Rest.