Whip of the Copper Eel (Patreon)

Weapon (whip), uncommon (requires attunement)

This magic whip hums with electricity. It has 4 charges and regains all expended charges daily at dawn. When you hit a creature with this whip, you can expend 1 or more of its charges to deal an extra 1d6 lightning damage to that creature for each charge you spend. You don't suffer the negative effects of underwater fighting when you make an attack with this weapon.

When you hit an underwater creature with this whip, it automatically takes an extra 1d6 lightning damage from the attack—as if you had expended a charge from it. When this happens, you can still spend charges from the whip to deal additional lightning damage to the creature. If you do, each other creature of your choice within 10 feet of the target must succeed on a DC 13 Constitution saving throw or take lightning damage equal to half the amount dealt to the target.

The merrow stared, slack-jawed, at the plate-armored figure stomping into their seabed sanctum. Its water-logged laughter stopped as soon as it started, cut short by the cold, measured gaze from behind the figure's frog-like visor.

Their amusement grew as the figure took a metallic whip from his belt: a worthless weapon for fighting in the merrows' home. It was almost pitiful, but… who were they to deny this fool the death his idiocy demanded?

Only the last of their squadron—a straggler behind the rest—lived long enough to catch the whip's true form, undulating like an eel, and arcing with a storm-like power that had torn through the others.