Agent of Plague. Any creatures entering an 80 ft radius of the Tetrodolite must make a Constitution 16 Saving Throw, contracting Tetrodolisis on a failed save, or becoming immune to this effect for the next 24 hours on a successful one. 

"It will stop at nothing to ensure plague unfurls across the world, that none can recover from sickness, to change the landscape for this sole purpose, The Agent of Plague."

Tetrodolisis. Creatures with Tetrodolisis will immediately gain one level exhaustion and vulnerability to poison, after 1d4 days, the rest of the symptoms manifest: The creature becomes weak to water, vomiting every 1d4 turns spent in it, dealing 1d6 acid damage. The creatures immune system also becomes weakened, causing Saving throws relating to the contraction and development (Manifestation of symptoms) of all diseases to be at disadvantage. Every time the creature ingests food, it must make a Constitution 12 Saving throw or vomit it up, dealing 1d6 acid damage and taking away all benefits gained from the food including satiation, on a successful save, the creature is immune to this effect for the next two times it eats. Every long rest, the creature takes 1d12 Psychic damage, and cannot recover it until it is cured. To cure itself, the creature must delve deep into the centre of the Tetrodolite's lair, where it's eggs are nested, and retrieve one, or part of it (10 kilograms is best). This must then be refined by pure magic energy in a pot over a flame, turning every 1d10 Kilograms into one dosage of the cure. If their are no eggs accessible, the creature can also retrieve an eye of a Tetrodolite, or part of it (30 kilograms is best) and go through the same process, turning every 3d10 kilograms of Tetrodolite eye into one dosage of cure. Anyone with Tetrodolisis cannot be cured of other diseases.

"Pure toxins flow through the victims veins, they scream in the agony of all those inflicted before them, swampy tears run from their eyes as they sit alone, where they will perish."

(Creator's Note to Dungeon Masters: If the party is too low a level to create a cure, just make it a purchasable item, don't kill them just for going too close to a Tetrodolite.)

Brawn Rot. A waterborne disease causing the deterioration of the muscles, the symptoms will manifest after 1d4 hours. The creature gains one level of exhaustion. The creature loses 1d8 Strength. Every long rest, the creature must make a Constitution 16 Saving Throw or lose 1d4 Strength, on a success the creature recovers 1d4 Strength, being cured if they return to their normal level of Strength, however if they have Tetrodolisis, the disease remains. Having a Healing spell cast with a spell slot of 2nd level or higher 1 hour before the long rest or gaining benefit from Song of Rest will give you advantage on your Constitution Saving Throw.

"As the lone survivor limps his way through the swamp, the rain rattling on the armour he had abandoned, he makes one last reach with his arm before he collapses, unable to move."

Coal Clod. A pathogen spreading from the mud and dirt of a swamp or other diseased region causing lumps to appear on the infected skin, the symptoms will manifest after 1d4 hours. The infected creature now requires doubled food and drink intake and will not gain benefits from short rests and will lose 1d4 Charisma. Every long rest, the creature must make a Constitution 16 Saving Throw or grow a multitude of diseased lumps across their body, losing 1d8 Charisma, upon success, they will recover 1d4 Charisma, if they return to their default Charisma, they are cured, however if they have Tetrodolisis, the disease remains. Having a Healing spell cast with a spell slot of 2nd level or higher 1 hour before the long rest or gaining benefit from Song of Rest will give you advantage on your Constitution Saving Throw. Every long rest, roll a d4, on a 1, one of the lumps on you skin will burst, causing a 10 ft radius circle to be contaminated with Coal Clod where you slept, anyone without Coal Clod entering the area must make a Constitution 16 Saving Throw or contract it, this effect also happens when an infected dies.

"She claws at the door, attempting to leave the place she had isolated herself in, trying to survive. Cold winds prod the door open, revealing a corpse riddled with busted black lumps, dripping into a pool of blood."

Legendary Resistance (3/Day). If the Tetrodolite fails a saving throw, it can choose to succeed instead.

Spellcasting. The Tetrodolite is an 18th-level spellcaster. Its spellcasting ability is Intelligence with a spell save DC of 16, +6. The Tetrodolite has following wizard spells prepared:
Cantrips (at will): Acid Splash, Chill Touch, Poison Spray, Prestidigitation, Ray of Frost

1st level (4 slots): Chromatic Orb, False Life, Hideous Laughter, Magic Missile, Ray of Sickness, Shield, Thunderwave, Witch Bolt

2nd level (3 Slots): Acid Arrow, Blindness/Deafness, Cloud of Daggers, Crown of Madness, Misty Step, Ray of Enfeeblement, Shatter, Suggestion

3rd level (3 Slots): Counterspell, Dispel Magic, Fear, Lightning Bolt, Vampiric Touch

4th level (3 Slots): Black Tentacles, Blight, Ice Storm, Phantasmal Killer

5th level (3 Slots): Arcane Hand, Cloudkill, Cone of Cold

6th level (1 Slots): Chain Lightning, Circle of Death, Disintegrate, Freezing Sphere

 7th level (1 Slot): Finger of Death, Prismatic Spray, Teleport

8th level (1 slot): Feeblemind, Power Word Stun

9th level (1 slot): Prismatic Wall, Time Stop, Weird

Fin Flip. The Tetrodolite is filled with rage, flipping it's tail towards it's target(s). Melee Weapon Attack: +14 to hit, 60 foot cone, up to 3 targets. Hit: 32.5 (5d10+5) slashing damage plus 26 (4d10+4) poison damage.

"The contaminated water moves with the Tetrodolite's tail as it unleashes this attack of sickening power."

Discharge Pestilence (Recharge 5-6). The Tetrodolite breaths in as it's insides glow an ever bright green, and then it releases pure toxic energy in a 100 foot cone. Each creature in that cone must make a Constitution 24 Saving Throw taking 66 (12d10) poison damage, then 33 (6d10) acid damage on a failed save, or half as much poison damage and no acid damage on a successful one. Any creatures hit by this must make a Constitution 18 Saving Throw, on failure, they contract Tetrodolisis.

"Sheer pestilence is unleashed to plague all those who stand against it, as the almost sappy acid and poison cover your body, you're consumed into a fate worse than death."  -Master Wizard, Archimage Dracnon.

The Tetrodolite can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Tetrodolite regains spent legendary actions at the start of its turn.

Legendary Discharge Pestilence (3 Actions): The Tetrodolite breaths in as it's insides glow an ever bright green, and then it releases pure toxic energy in a 100 foot cone. Each creature in that cone must make a Constitution 24 Saving Throw taking 66 (12d10) poison damage, then 33 (6d10) acid damage on a failed save, or half as much poison damage and no acid damage on a successful one. Any creatures hit by this must make a Constitution 18 Saving Throw, on failure, they contract Tetrodolisis. This action requires recharge. This action is legendary, meaning that this version of the action is recharged separately from it's counterpart.

Recharge Legendary Discharge Pestilence (3 Actions): Legendary Discharge Pestilence may be reused after this creature's next turn ends.

Fin Flip (2 Actions): The Tetrodolite is filled with rage, flipping it's tail towards it's target(s). +14 to hit, 60 foot cone, up to 3 targets. Hit: 32.5 (5d10+5) slashing damage plus 26 (4d10+4) poison damage.

Cunning Action: The Tetrodolite may Dash, Disengage or Hide.