Leviathan

Leviathans are bipedal whale-like creatures corrupted by the Olden Ones. Cruel and warlike, they are feared by sailors and other aquatic races, seen as the demons of the seas. They have voracious appetites and eat anything they can find. All leviathans share a single stomach, a realm known as the Gastrosphere.

Traits

Ability Score Increase. Your Constitution score increases by 2, and your Strength score increases by 1.

Age. Minnow-like infants spend 1,000 years swimming in the sea before growing into adults. Leviathans can live in adult form up to 10,000 years.

Alignment. Leviathans seek to upend the status quo and would see the world drowned if they were able. They have abused the Eternal gift of Essence since before recorded history. Leviathans are chaotic evil.

Rarity. Mythical.

Size. Leviathans are massive, between 7 and 8 feet tall and weighing up to 600 pounds. Your size is Medium.

Speed. Your base walking speed is 30 feet. You also have a swimming speed of 30 feet.

Darkvision. You can see in dim light within 60 feet of you as if it were bright light, and in darkness within the same range as if it were dim light. Absent even dim light, this vision is monochromatic.

Bite. You have fangs, and so your unarmed strikes can deal ld4 piercing damage. Your bite has the light and finesse weapon properties, and you have proficiency with it.

Grappling Tongue. You can use your tongue as a free hand to grapple.

Heritage (Fiendish). You are humanoid, but you are considered to be a fiend whenever it is detrimental for you.

Hold Breath. You can hold your breath for a number of minutes equal to 20 x your Constitution bonus. Once you run out of breath, you can survive for a number of minutes equal to 1 + your Constitution bonus.

Ocean Dweller. You are inured to the pressure and temperatures of the cold depths. Add your proficiency bonus to any saving throw against extreme cold or the pressure of deep water. If you have proficiency in such a saving throw, your proficiency bonus is doubled for these purposes.

Water Jet. As an action, you create and launch a jet of water at one target you can see within 30 feet of you. The target must succeed on a Strength saving throw (DC = 8 + your Constitution modifier + your proficiency bonus) or be pushed up to 10 feet away from you. If you target an unsecured object that is neither worn nor carried and is Medium or smaller, it is automatically pushed.

Languages. You speak, read, and write Common and Aquatic. Many leviathans also learn to speak, read, and write Void.

Leviathan Ascension. When your character finishes a campaign, you can work with your GM to make the character a deific otherworldly NPC. If your GM agrees, your leviathan ascends and becomes a patron of chaos and evil in the campaign world, possibly acting as a source of power for and influence on future PCs. The NPC leviathan acts much as you roleplayed your character during the campaign.