Dekella, Bident of Thassa

Weapon (trident), artifact (requires attunement)

Thassa wields Dekella, a two-pronged weapon gifted to her by Purphoros. When the god of the sea bestows her weapon on a mortal, it's often so they might work her will far from the ocean, right some wrong affecting those she'd prefer not to harm, or give a mortal a glimpse of her terrible responsibilities.

Bident of the Deep. Thassa's signature weapon thrums with the icy currents of the deep ocean. This weapon functions as a trident that grants a +3 bonus to attack and damage rolls made with it. When you hit with an attack using the bident, the target takes an extra 2d10 cold damage.

Command the Deep. The bident holds the power to command the waves and its creatures. As an action, you can change the condition of the sea within 1 mile of you, creating strong winds and heavy rain that cause violent waves (see "Weather at Sea" in chapter 5 of the Dungeon Master's Guide) or calming a storm. In either case, the unnatural weather lasts for 1 hour before returning to normal. Once used, this property of the bident can't be used again until the next dusk.

Blessing of the Deep. If you are a worshiper of Thassa, you gain all the following benefits for which you have the required piety:

Piety 10+. You can breathe underwater, and you gain a swimming speed of 60 feet.

Piety 25+. The bident has 1 randomly determined minor beneficial property.

Piety 50+. The bident has 1 randomly determined major beneficial property.

If you aren't a worshiper of Thassa, the bident has 1 minor detrimental property and 1 major detrimental property, both randomly determined.

Aquatic Master.

You must be attuned to Wavemother's Robe and Wavemother's cloak for the following effects. You can cast the True Polymorph spell (save DC 18) from the bident, but you must cast it on a creature to turn it into a kind of creature that has an innate swimming speed. Additionally, you can cast the Dominate Monster spell (save DC 18) from the bident, but only on beasts and monstrosities that have an innate swimming speed. You can summon a water elemental as if you had cast the conjure elemental spell. Once used, the same effect can't be used again until the next dusk.