Wavebreaker

Weapon (heavy crossbow, requires attunement), legendary

Wavebreaker is a +1 heavy crossbow without the loading trait. It is typically carved from whalebone or the shell of a titanic mollusk or crustacean, decorated with intricate scrimshaw and nacreous inlay and with a bow and mechanism of gilded steel or similar rust-resistant metal. Ammunition shot from a wavebreaker is antithetical to any aquatic or watery creature (any creature with a swim speed).

If an aquatic creature takes damage from an arrow shot from the wavebreaker the creature must make a DC 17 Constitution saving throw, taking an extra 6d10 piercing damage on a failed save, or half as much extra damage on a successful one. This bane effect once used can’t be used again until the next dawn. In the meantime, the wavebreaker can still be used as a magic weapon. Additionally, water does not provide any situational benefit (for example extra cover or disadvantage) against attacks made with a wavebreaker.

Finally, you can target a creature or spell that is watery by nature within 30-feet as an action.

You automatically hit if it is a spell. If the target is a creature you must make a normal attack roll against the creatures AC. On a hit, wavebreaker deals no damage but acts as dispel magic against the spell or a banishment against non-native creatures (for example a summoned water elemental). You can use this ability as often as desired but a creature or spell that is not dispelled or banished becomes immune to this effect for 24 hours.