On Fish-People

I have long complained that, of all the array of fish-people and seafolk available in D&D, they're all kind of samey. Let's see, we've got...

• Merfolk are people with a fish tail
• Tritons are people who live underwater and are just kind of slightly fishy -- in earlier editions, they were at least Outsider(native)s, which is something to latch on to. (Although it does make them kind of overlap with water genasi -- but without it, they instead overlap with, like sea elves.)
• Locathah are just kind of generically fishy, although in earlier editions they kind of looked like the Creature from the Black Lagoon
• Sahuagin are just kind of generically fishy, albeit with a telepathic connection to sharks
• Kuo-toa are just kind of generically fishy, albeit also slimy

So let's see what I did with that to make it a little less boring (and also options for player characters, because my setting is so heavy on seafolk)...

Merfolk

Species Traits

• Ability Score Increases: Your Dexterity score increases by 2, and your Charisma score increases by 1.
• Type: Your type is Humanoid.
• Size: Your size is Medium.
• Speed: You have a land speed of 10 feet and a swim speed and a swim speed of 40 feet. You begin play with a free Combat Wheelchair, which grants a land speed of 25 feet.)
• Amphibious: You can breathe both air and water.
• Versatility: You gain one free skill proficiency of your choice and one free tool proficiency of your choice.
• Languages: You can read, write, and speak Common and Aquan.

Triton

In previous editions, tritons were native outsiders. In mine, they're still immigrants from the Hellish Plane of Water. So...

Species Traits

As in Mordenkainen Presents: Monsters of the Multiverse, except:

• You have the Humanoid and Elemental types. If any favorable effect would affect either of your types, it affects you. If any unfavorable effect would affect one of your types but not the other, you can be affected by it but you have advantage on saves against it. If any effect would affect neither of your types, you are immune to it. If any effect would affect both of your types, you do not gain immunity or resistance to it from your types. 

Sahuagin

Notwithstanding recent efforts to make them, like, green and lizardy, Sahuagin can just be shark-people.

Species Traits

• Ability Score Increase: Your Strength score increases by 2, and your Wisdom score increases by 1.
• Type: Your type is Humanoid.
• Size: Sahuagin are larger than humans, ranging from 6 to 7 feet tall. Your size is Medium.
• Speed: You have a base land speed of 30 ft, and you have a swim speed equal to your land speed.
• Superior Darkvision: Within 120 feet, you can see in dim light as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.
• Limited Amphibiousness: You can breathe both air and water. However, you cannot receive the benefits of a Short or Long rest unless you are fully submerged in water for the entire duration of your rest.
• Frenzy: On your turn when you reduce a creature to 0 hit points, you can use a bonus action to move up to your speed and make an additional weapon attack against another creature you can target.
• Shark Telepathy: You can telepathically speak to any shark within 120 feet of you.
• Claws and Bite: Your claws and bite are natural weapons, which you can use to make unarmed strikes. If you hit with them, you deal slashing (claws) or piercing (bite) damage equal to 1d4 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.
• Languages: You know Common and Sahuagin.

Kuo-Toa

Kuo-toa shall be anglerfish, complete with anglerfish sexual dynamics: 

• Males are Tiny Beasts with 1 Intelligence (using the Quipper statblock).
• Females are Medium Humanoids (using the Kuo-Toa statblock, or the species traits here).

Biologically female Kuo-Toa have six genders (Up, Down, Charm, Strange, Top, and Bottom), which they borrowed from the sexual hexamorphism of the Aboleths they chill with. It's not well-understood by surface-dwellers how romantic combinations of these genders works.

Species Traits

• Ability Score Increase: Your Strength score increases by 2. Your Constitution score increases by 1.
• Type: Your type is Humanoid.
• Size: Your size is Medium.
• Speed: You have 30 foot land speed and swim speed equal to your land speed.
• Superior Darkvision: Within 120 feet, you can see in dim light as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.
• Amphibious: You can breathe air and water.
• Sunlight Sensitivity: You have disadvantage on attack rolls and Wisdom (Perception) checks that rely on sight when you, the target of the attack, or whatever you are trying to perceive is in direct sunlight.
• Otherworldly Perception: You can sense the presence of any creature within 30 feet of you that is invisible or on the Ethereal Plane. You can pinpoint such a creature that is moving.
• Slippery: You have advantage on ability checks and saving throws made to escape a grapple.
• Bite: Your bite is a natural weapon, which you can use to make unarmed strikes. If you hit with it, you deal piercing damage equal to 1d4 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.
• Languages: You know Common and Kuo-Toan.

Locathah 

I decided Locathah should be axolotl-folk.

The main deal with axolotls is that they are actually the tadpole form of a salamander which just never metamorphoses, just grows to an adult size instead. Thyroid hormone can be administered to induce metamorphosis into a salamander that hasn't been seen in the wild for millions of years. So, naturally, axolotl-folk can do that with magic.

Species Traits

As in Locathah Rising (Extra Life 2019), except:

• Replace the listed Limited Amphibiouisness trait with:
• Limited Amphibiousness. You can breathe both air and water. However, you cannot receive the benefits of a Short or Long rest unless you are fully submerged in water for the entire duration of your rest.

Feat: Metamorphosed

A Locathah shaman can cast the Ceremony spell on another Locathah to induce metamorphosis into a super-adult stage.

Prerequisites

• Locathah species.
• You must be under the effects of the Coming of Age Ceremony cast by a Locathah shaman at time of feat selection.

Effect

• You lose the effect of your Limited Amphibiousness trait, and are now fully amphibious -- you can breathe both air and water, and do not need to be submerged in water to gain the benefits of a Short or Long Rest.
• You have Resistance to Poison damage.
• Your skin turns a vivid color and begins to secrete a poisonous toxin. If you are hit by a Bite attack, the attacker takes 1d6 Poison damage. If you are subject to the Swallow Whole ability, the creature that swallows you takes xd6 poison damage on your turn every round you are swallowed, where x is equal to your Constitution modifier (minimum 1).
• You can harvest your own poison as per the rules in the Dungeon Master's Guide, except you do not need to be incapacitated or dead; you are not immune to your own poison, but you remain Resistant to its Poison damage. The harvested poison can be used as-is or refined:
• When used as-is, this is an Ingested poison that deals 2d6 Poison damage, Constitution save negates. A dose of this poison lasts for 24 hours before it becomes inert.
• Refining the poison follows the Crafting rules, but you have unlimited access to the ingredients. When refined, it becomes a dose of Pale Tincture poison, which lasts indefinitely until use.

The Year Hoobynoob Unaccountably Contrived to Come on Freday the 13th of Octember

Through some celestial mishap or accounting error, for some reason, Hoobynoob falls on Freday the 13th of Octember, in this, the 47th year of the Subsidence. Normally, of course, Hoobynoob falls on the 30th, the last day of Octember. The 13th of Octember doesn't normally even fall on a Freday -- it is typically a Munday.

This calendrical irregularity is known for certain: all the experts agree on this unusual occurrence -- the astrologers; the astronomers; the druids and rangers of Sequoia (who counts the months and weeks, the planets and stars, under her purview); the calendricians and calendrometers; we even asked the janitor, as they know of these things.

Amongst the populace, once the confusion is overcome -- or at least set aside -- the yearly observance of Hoobynoob proceeds as usual, albeit on the 13th. In Endeesy, capital of Gus: gourds are carved; nuts are harvested; costumes are prepared; candies and sweets are purchased or crafted in bulk in preparation for the traditional mass extortion, on threat of prankery, by children.

When the 13th of Octember arrives -- on a Freday, just as (unusually) ordained -- the morning sneaks into Endeesy foggily. The day slugs by: children growing increasingly excited, adults growing increasingly wary of danger, the weather growing increasingly foggy and moist-smelling.

When treat-or-tricking time arrives, it is nigh impossible to find one's way in the fog. But, through some divine Dalyan providence, unusually alert priests of Dalya, and widespread use of the Buddy System, no children are separated from their guardians for long.

The children return to their homes, feast on junk food, and repair engorgedly to their beds. After the foggy day and foggier evening, it looks like it has been another successful Hoobynoob, profitable for the children and fun for all ages! Parents go to bed. Bar crawlers begin the process of acquiring their holiday drunkenness.

Midnight approaches.

The clock tower in the Cathedral of Quasxthe in the Crown District of Endeesy ticks over from 11:59, and the mighty bell begins its twelvefold tolling: BO-o-...

The bell, having tolled half of once, trails off, as if it has forgotten how to bell.

Mournful moans are heard through the increasingly farty-smelling fog.

Zombies! Some of whom are fresh, some of whom are decaying and dirt-encrusted.

Following the trail of zombies leads the party north, to a graveyard in the Resentment Ward. The zombies are discovered to be animated by worms squirming under their skin (a miniboss zombie acts as Spawn of Kyuss).

A 400-year-old mausoleum, dating to the early centuries of the Inundation, proves to be the resting place of the Mermaid Queen Riefya I Arielid "the Black".

A party member recollects a fact about Riefya "the Black": that she was found, after her death, to have secretly been a cultist of Asya, goddess of all that is disgusting.

The lock on the door, being 400 years old and having been underwater most of that time, has become more of a jam. But somebody has already broken the jam. But somebody subsequently cast an arcane lock on the door. But the rogue breaks the arcane lock.

Within the tomb, reality is distorted. The air is fetid and moist. The stairs down are greasy. The walls are fleshy and moist. It is not a throat, but it desperately wants to be a throat.

At the bottom of the stairs, the tomb of Riefya has expanded from a small room to a large acid-lake cavern, as if it wants to be a stomach.

A sarcophagus stands on a raised dais in the middle of the acid lake. A mage stands by the sarcophagus, incanting a ritual.

He is Vyndoc Hogkt, a descendant of the Hogkt family that ruled the Asya-worshipping Plaguelord nation while it existed in the first century before the Inundation. (He is a Mage, with spells swapped out for poison- and acid-related spells.)

His ritual concludes, and an oozely incarnation of Asya possessing the corpse of Queen Riefya joins combat. (She uses the stat of a Black Pudding, except she can throw acid gobbets -- her Pseudopod has a 60' range.)

If the party kills Riefya!Asya first, Vyndoc sacrifices himself to regenerate and empower her. If the party kills Vyndoc first, Riefya!Asya eats his corpse and is regenerated and empowered. Either way, it is a two-stage boss fight. (The amalgamation boss has a combination of the abilities of its components, and then some.)

The party defeats Vyndoc, Riefya, and Asya, and prevents Bad Things from happening.

Time progresses from midnight once they emerge from the tomb.

Hoobynoob did not come on Freday the 13th because of Vyndoc (that's an unsolved mystery), but since it did, and both 13 and Hoobynoob are sacred to Asya, he took it as auspicious to enact his plan.

How the Bonelord Stole Driftmas

On the last day of the year, three Guildmasters of the Endeesy Adventurer's Guild -- Ruthmin Tern the Assassin Rogue, Christopher the Paladin, and Lyra the Stars Druid -- were chilling in the Guildmasters' Lounge when seven driftmas fey popped through a portal, begging for their assistance. It turned out that Saint Usla, in charge of Driftmas, had been kidnapped by fire creatures.

The Guildmasters tracked the scorch-trail from the Driftmas Demiplane, to the Transitory Holiday Demiplane, to the Hoobynoob Demiplane.

They found (and broke into) a laboratory guarded by salamanders, where Saint Usla had recently been, but she (and the Professor, one of Hoobynoob's people) had already been stolen by disgusting creatures.

In the laboratory greenhouse, they found a construct named Sam, who was very concerned that Jaq the Gourd Duke, in charge of Hoobynoob, had been acting very strangely, including with this peculiar plan for Saint Usla.

They went to Jaq's mansion, where they were hired by Jaq to recover the Professor. Jaq's theory was that Neepkag the Bogeyperson, who had been in charge of Hangadh-Buain-Noiche before the Burning Hate took it from Asya and put Jaq in charge as the Incandescent Bonelord of Heckbound Night (long before Dalya took over, renamed it to Hoobynoob, and made Jaq the Gourd Duke), wanted to carry out Jaq's own plan, but to their own benefit.

(Jaq's plan was to split his two personalities -- the Incandescent Bonelord following the Burning Hate, and the Gourd Duke following Dalya -- putting one into Saint Usla, so that each could be in charge of their own holiday. The Professor had all the details, it involved a plant that had been grafted with black dahlias and orange lilies, which through the laws of similarity could split the two personalities if ungrafted, and then graft the orange lilies onto a sunflower, representing Saint Usla's Numiel.)

The Guildmasters went to the Bogeyperson's cave, defeated them, rescued Usla, and were declared Nice and deserving of many presents by Usla. They returned the Professor to Jaq, who now owes them an unspecified favor.