This is my response for the Chaos Request Line, which is sort of like a year-round Santicore. This is in fulfillment of +Daniel Dean's request, who wanted a race-class that wasn't based on a specific animal, element, environment, or mechanic, and who isn't a construct.
Unlike most cases, this race-class is not determined at character creation. The only way to become a fleshgod inheritor is to venture into a dungeon, museum, or royal vault, and find a canopic jar of the fleshgod.
These are 2' tall canopic jars that contain the organs from a fleshgod. These organs resemble their human counterparts only vaguely, and are about twice the size. The organs are immortal; inside their jars they spasm and spurt, exactly as they have been doing for the last century.
Fleshgods are star beasts of such exceptional puissance that they seem to be nearly impossible to kill. Once they fall to earth, they are too weak to climb out of their craters, and will merely lie there, wailing and bleeding, flopping around helplessly under our gravity. Eventually, they will be eaten by insects (the only creatures immune to the fleshgods' psychic blasts, as they lash out indiscriminately as they are being devoured over the course of years).
Only two fleshgods have ever fallen: Belpharukh and Gilahn. The fate of Belpharukh is well-known and outside the scope of this page.
Gilahn was revered as a god for almost a decade before the cult was massacred and their relics dispersed. Eventually the great god-corpse was cut up and used to feed the ghoul armies that marched on Yog.
Some of the relics that escaped the destruction were the canopic jars of the fleshgod, which contained its most potent organs. They are now scattered across the world, mostly in dungeons. When a PC finds a canopic jar and consumes the contents, they lose all XP and become a level 1 Fleshgod Inheritor.
Alternatively, a canopic jar can be sold for 1000s.
A new race-class who isn't keyed to a specific element or environment, who isn't patterned after a specific animal, and who isn't a construct.
Basically, take a human cleric and replace Turn Undead with Cosmic Cyst (below). Then replace all the spellcasting with Mighty Godflesh (below). Additionally, they can sense when a dungeon/building/hex contains an unopened canopic jar of the fleshgod.
One caveat: In order to level up as a fleshgod inheritor, a PC needs both the requisite amount of XP and to consume a new piece of fleshgod flesh from a canopic jar. When a Fleshgod Inheritor gains enough XP to approach the next level, the DM should insert a canopic jar somewhere into the current (or next) dungeon. Not at the end or the beginning, but somewhere in the middle. Roll 1d20 to pick a room if you have to.
Inheritors who spend time tracking down rumors and lost bits of lore should be able to learn the locations of specific types of organs, so that they can guide their evolution as they see fit. (See Mighty Godflesh, below.
One last caveat: when the Fleshgod Inheritor reaches 10th level, they begin doubling in size every day, instructed and fatted by extracosmic energies. Additionally, each day they must make a save or become an NPC, compelled to fly away into space in order to achieve their destiny.
Tear open a gateway to a cosmic cyst, a biomechanical chamber embedded in the substrate between the voids. (Physically, it resembles the inside of a garbage bag arranged in the shape of a derelict machine shop, through which a rain of H2O falls in slow motion.) Functionally, it works like the rope trick spell, giving beleagered parties a secret place to rest that is inaccessible to their enemies. After 8 hours of use, the cyst begins its digestion cycle, dissolving anything inside that doesn't exit immediately. The cosmic cyst is already in used by another fleshgod inheritor of HD 1d8+2 (50% chance) or 1d4+1 fleshgod inheritors of HD 1d6+1, who is either present when the party first enters or who arrives shortly thereafter. Once the rival inheritor is slain, the cyst is indisputably yours. If you invade their home and then flee, your rival fleshgod inheritors might chase you back into your dimension (but they often prefer to avoid our caustic atmosphere with all it's filthy nitrogen).
Everything in the previous paragraph is vaguely known to the nascent Fleshgod Inheritor. While they can access their cosmic cyst from level 1, most of them are smart enough to avoid entering it until they are confident they can overcome its guardian.
There are six types of canopic jars, each holding a different type of godflesh.
Each time you eat an organ of the fleshgod Gilahn, you either gain a new ability or upgrade an old one. These upgrades are non-stacking: once you learn Heart Ability #1, you lose access to Heart Ability #2.
Hearts ooze black ectoplasm for as long as the are whole. They are usually found in the bottom of chests that eternally leak what looks like engine oil. They taste like bitterness and old age; your saliva dissipates into grey smoke.
The first time you eat a fleshgod heart, your skin becomes covered with fleshy spines. You can no longer wear armor, but get protection as if from leather armor (but it is as light as no armor at all). Additionally, you can rattle your spines together, which causes humans and halflings within 100' to save vs fear or flee.
The second time, your skin becomes covered a shifting membrane that reflects a foreign starscape. You still can't wear armor, but it functions as chain armor (but it is as light as leather).
The third time, your rib cage swells, bursts, and hardens into a huge carapace. The shell hunches your back and arches over your head. You still can't wear armor, but it functions as plate armor (but it is as light as leather). Additionally, you have a 20% chance to reflect any spell cast on you, even helpful ones.
Lungs flutter out of their broken jars like sodden moths. A mad piping emerges from their curled tubules. They taste like hard vacuum, a pucker that travels down to your feet and makes your blood pool around coccyx.
The first time you eat a fleshgod lung, you gain two level 1 spell slots that you can each use 1/day. The first slot holds immanentize the eschaton and is immutable. The second slot is empty, but can be filled by a spell of your choice. To fill the slot, you must obtain a level 1 spell scroll, then cut open your belly button and stuff the scroll directly into your stomach. While you sleep, the sound of the ocean comes from your guts (as if you swallowed a speaker).
The second time is as above, except that you have two level 2 spell slots, the immutable one containing wave of extinction. While you sleep, the sound of a forest fire comes from your guts.
The third time is as above, except that you have two level 3 spell slots, the immutable one containing homeward disjunction. While you sleep, what sounds like whalesong comes from your guts.
Livers invariably outgrow their canopic jars. They feed on extradimensional poisons that drift through the ether. If a liver escapes its jar and is fed a steady diet of poison, it can grow large and feral, eventually becoming a Fleshgod Hepatoprax (see below). If eaten, it tastes like blood (only moreso) and causes your teeth to loosen in their sockets and the striations of your muscle to be visible beneath your skin.
The first time you eat a fleshgod liver, your primary hand turns into what looks like a black fractal antler. It shudders as it instantaneously grows, branches, and collapses. It deals 1d6 damage, as a sword. On a crit, it drains blood (if the target has blood) healing you for 1d6 HP. This also causes the whites of your eyes to turn red for several minutes until the blood clears.
The second time, your primary hand turns into a serrated claw. It deals 1d8 damage, as a heavy weapon. On a crit, it rips the armor off an armored opponent (if logically plausible) and hoarse cries of triumph in an alien language burst from a hidden throat someone in your guts.
The second time, your primary hand turns into a tentacle with an oddly bony tip. It is snapped like a whip, and is powerful enough to snap logs in half. It does 1d10 damage. On a crit, the target is disemboweled, and their intestines fall out of them like ground beef out of a styrofoam cup with the bottom cut off. (Vorpal, essentially.) Additionally, you are consumed with a powerful desire to eat a piece of each creature you kill, and must make a Cha save to resist this urge.
Brains are have the familiar sulci and gyri of human brains, but are shaped like traffic cones. Anyone looking directly at them has all the colors in their vision red-shifted. Brains taste fizz and pop while they dissolve on your tongue, and they taste like ozone and acid burns.
The first time you eat a fleshgod brain, you gain the ability to swallow opponents: make an attack roll against an adjacent opponent. If that opponent has 3d6 or less HP your jaw dislocates, distends, and you swallow them whole into the caustic oblivion that lines your soul. The target and all of it's possessions are usually unrecoverable. That night, you spend a couple minutes painfully vomiting out all of their bones. Each of their items has a 2-in-6 chance of also being ejected at this time; otherwise, it is lost forever.
The second time, you gain the ability to become a travelling tumor: You can enter a willing creature, existing as a tumor on their back. Mechanically, treat this as riding a mount, except that you are fused with them and weigh about 1/5 your regular weight. While so fused, you can still speak and use your hands, as normal. You and your host now share an HP pool, and you can redistribute HP as you see fit. This also allows you to move damage between willing targets, as the donor's lacerations seal with slurp and the recipient's chest suddenly blossoming with sucking chest wounds, like mushrooms after a rain. You can exit your 'mount' at will.
The third time, you gain the ability to lay paradox eggs: You lay an egg. As long as it is kept watered with blood (1 gallon a day) it will live. If the egg dies, anyone who gazes at the inchoate fetal paradox within must save or gain an insanity, and you will lay a new one in 1d12 months. If you die while the egg is alive, your inheritor will hatch from the egg immediately. The hatchling is you! It has all of your memories and skills (although its mind is now speckled with visions of the interplanar gulf and the creaking abysms of creation). So use your previous character sheet, with no XP loss or anything. (You were inside the egg. You were always inside the egg, even as you laid it. It is a paradox.) You have -4 to hit while you are still so small, but you grow to full size quickly, reducing this penalty by 1 per week as long as you eat a large amount of food daily (a small cow will do), potentially returning to full strength as quickly as a month.
Stomachs are rugose like a cross between brain coral and unstretched leather. Unlike their peers, they are slight and fearful organs, and will cower in the bottom of the canopic jar, mewling in terror, recoiling from your touch. They taste like rupture and transgression, a cross between a popping balloon and kissing your mom.
The first time you eat a fleshgod stomach, you get a freckled pot belly. Not an ugly pot belly, a cute one. Your other limbs narrow to aesthetic proportions. Freckles travel across the pot belly like emotive stars. Additionally, gravity is now reversed for you. (Hopefully you ate the stomach indoors.) If you want to walk on the same floor as everyone else, you can weigh yourself down with weights or a specially made suit of plate mail, but this is unwieldy and gives you -2 to hit. Additionally, you are immune to cold and vacuum, and can survive comfortably in the void of space until you comfortably starve to death.
The second time, your eyeballs drift out of your head and begin orbiting it like IOUN stones. Then another pair of eyes flies out of your sockets and joins the first, then another, and another. You can see in all directions as easily as you can see in one. However, you can only see through your 8 eyes as long as they remain in orbit around your head. You can climb on things, as if permanently under the effects of spider climb.
The third time, your belly button vanishes. Anything you say is now echoed by a basso voice located in the vicinity of your stomach. Additionally, you can use dimension door 1/day, but you can only travel to and from a place where many sharp angles intersect (such as the corner of a room).
Intestines loop in on themselves, building a mobius digestive track. Annular muscles push grey sludge through the loop in peristaltic waves, making a sound like a water heater full of mineral deposits. (You do not have to eat the grey sludge. If you do, save vs poison or suffer vomiting and weakness (-2 to hit) for rest of the day.) The stomach tastes like broken teeth and bruised molasses. You don't swallow it so much as allow it to slither down your throat.
The first time you eat fleshgod intestines, you gain an organ in your throat that gives you echolocation, allowing you visualize all solid surfaces within 30'. It is very noisy; it sounds like a vacuum cleaner trying to suck up a dead cow at a foghorn party.
The second time, your eyes sink into your skull, becoming inverted cones with a depth far greater than your skull would normally allow (non-Euclidean spaces) filled with a mist that shares your original eye color. This gives you X-Ray vision (stopped by foot of stone, inch of metal, mm of lead) for solid objects, but you can no longer discern colors (at least, not colors that your peers would understand) and so can no longer read books.
The third time, you midsection disappears entirely. Your upper body ends at your ribs, which now have skin, even on the bottom. Your lower body ends at the hips, which now have skin, even on the top. You cannot separate these halves any more than you could before--your torso still exists, merely in another dimension. Blood flows from your heart to your feet as normal; it just passes through another dimension to get there. Additionally, you gain the ability to travel back six seconds in time with full knowledge of what transpired (usable once per day). This ability is 90% reliable, but each time you use it, the chance of success drops by 10%. This chance is only increased by eating the viscera of a sentient creature (which increases the chance of success by 1% per HD). If this roll is failed, not only do you fail to travel back in time, but a paradox clone of yourself is created somewhere in the world that has all of your knowledge and abilities (but not equipment) and will work tirelessly to thwart all of your goals.
Immanentize the Eschaton
Level 1 Wizard Spell
The sky turns an unnatural color, crows fly backwards, or slain animals laugh in your nets. This spell hastens the apocalypse by 1 day, and there are entire cults dedicated to its casting. Additionally, it has a useful side effect: for 1 hour, all clerical and divine magic cast within 50' (of this spell's point of origin) is weakened, and all creatures get a +2 bonus to save vs divine magic.
Chain of Extinction
Level 2 Wizard Spell
If the target fails a save, it takes 3d6 extinction damage, and this spell immediately jumps to the next nearest target of the same species as the last target. Creatures killed by this spell immediately decay into gravedirt and fused fossils.
Level 3 Wizard Spell
Target takes 1d6 damage for every 100 miles it is from home. Save for half. If they fail the first save, they must make a second save, or be returned to the location of their birth.
AC has leather
Movement as human
Bludgeoning tubules 1d6/1d6 + hemorrhage
Special: Gallstone 1/day. Explodes in a 20' radius on contact (similar to fireball) that cuts target's current HP in half. Save for half (so drops current HP to 75%).
Special: If both tubules hit the same target, they begin hemorrhaging, filling their eyes with blood (blinded) and dealing 1d6 damage per turn. Lasts 1d6 rounds or until they receive healing (magical or otherwise).
They look like bear-sized livers that slide around on a trail of their own blood. Although they have four limbs (and are capable of walking) they prefer to slide on their "bellies" and use their four radial limbs to shove themselves around.