Riptide Katana (Patreon)

Weapon (longsword), uncommon (requires attunement)

This item appears to be a longsword hilt. While grasping the hilt, you can use a bonus action to cause a blade of silently churning water to spring into existence, or make the blade disappear. While the blade exists, this magic longsword has the finesse property. Despite the nature of this watery blade, it cuts through material like any other sword. A fire elemental or similar creature made of fire that's hit with the weapon takes an extra 5 cold damage from the attack.

While the weapon is on your person, you have a swimming speed of 30 feet. If you make an attack with the weapon while underwater, you're considered to have a reach of 15 feet with the attack. In addition, the first time on each of your turns when you hit an underwater creature with this weapon, you can choose to move it 5 feet in any direction.

There were rumors of a water spirit that haunted the docks at night. A harmless beggar, bearing merely a tattered cloak and bladeless sword handle, would warn passersby of the watery threat, shouting "Beware the moonlit tides! The ruthless spirit of the deep claims all lives!" It seemed to keep to the old section of the city, and for the most part only attracted fools looking to tempt fate.

It was a thriving city, though, with many fools to spare.

Those who sought the spirit out would find it easily enough. It would float just beneath the water's surface—its dark form looking all the more phantasmal through the rumbling tide. Those who saw it, peering over the dock's edge, were met with a swift and piercing stream of brine before they were pulled down into the unforgiving drink.