Fleshcrafting, also known as Fleshwarping, is a term used on /tg/ as a catch-all to a specific form of dark magic and/or mad science that revolves around augmenting, transforming, shaping, sculpting and creating life, typically through a horrific amalgamation of alchemy, surgery and, if present, sorcery. Common uses of fleshcrafting include producing grotesque grafted biological augments or weapons, and creating monsters from corpse-pieces or living beings, usually to serve as servants of some description. This field of interest is usually restricted to races and individuals of the evil NPC variety, as it is generally seen as being too morally reprehensible (or overpowered) to allow PCs to have it.
Fleshcrafting has been used in the background of various monsters and even races since Dungeons & Dragons was first a thing. The Flesh Golem, the Broken One, the Bulette, the Owlbear and many more are usually said to be the result of mad wizards or dark artificers playing around with the secret of life. Heck, even Mongrelfolk have been portrayed as fleshcrafted once or twice; the Darklord of G'henna has the ability to transform worshippers of Zhakata he doesn't like into mongrelfolk.
In Advanced Dungeons & Dragons 2nd edition, PCs were pretty much restricted from using this style, although the Anatomist kits for the Necromancer (Wizard) did eventually allow the player to build flesh golems and, technically, the Xixchil could touch upon it through its racial ability to self-improve by grafting on armor and weapon-limbs. Several unique NPC monsters also existed to represent victims of flesh-crafting experiments.
In 3rd edition, PCs could actually get involved in this themselves. The principles of creating grafts were first introduced in the 3e Fiend Folio, then expanded upon in Lords of Madness, which presented the Graft Flesh item creation feat, several new aberration grafts, and the Fleshwarper, a Prestige Class specialized in augmenting itself and others by grafting on bits of defeated monsters. The Half-Golem template technically represents one branch of fleshcrafting study. A Major Artifact in the Book of Vile Darkness splatbook called "The Despoiler of Flesh" is a rod made from human tongues sewn together, which grants its wielder the ability to mutate others more or less as they see fit. Fleshcrafter was also a class presented in Dragon Magazine.
It appears extensively in the Eberron setting, being the specialty of the Daelkyr and their minions, and the source of all aberrations in the setting. Even one of the races called out as masters of grafting and flesh crafting in d&d, mindflayers, are a product of flesh crafting. They run the gamut of the standard grafts seen in the fiend folio, cool fleshy armor, a ball of meat that mutates anything you put in it, and whole new races made of the various goblinoids. A few other factions get in on the fun, but fleshworking is the Daelkyr's specialty and they're better at it than anyone else.
In Pathfinder, several archetypes for the Alchemist make use of Fleshcrafting, whilst it is an "artform" mastered and wielded by both the drow and the derro. The drow in particular have created a number of unique, horrific monster species through their established procedures for warping living organisms to their liking.
The Ravenloft fan-wiki divides Fleshcrafting into two branches; Biomancy (magical & psionic fleshcrafting) and Bio-Engineering (mad science fleshcrafting).
In Eberron fleshcrafting is mostly associated with Dealkyr and their insane cultists, responsible for the creation of most aberrations and a sizeable chunk of monstrosities in the world. Notably, some of the classic aberrations like Chokers, Neogi or Beholders in Eberron are products of Dealkyr fleshcrafting, as are Dolgrims and Dolgaunts, which are iconic aberration mooks of Eberron. Some Dorfs of Mror Holds took fleshcrafted weapons and symbionts as trophies from their fights with the legions of fleshcrafted aberrations during their ongoing war to retake their ancestral Realm Below, and some of the clans appropriate the art of fleshcrafting to fight the enemy with their own weapons in the best traditions of Radical Inquisitors. Some clans like Soldorak and Narathun embraced fleshcrafting whole-heatedly, cladding their elite soldiers in living armour and arming them with symbiotic weapons, even creating living equivalents of most popular magic items (symbiotic items are easier to create and they even reproduce naturally, so they're cheaper, but they drink your blood/hit dice every day to sustain themselves and can only be detached from you with magic, so there's pros and cons), seeing it as just another powerful tool no better or worse than metal or magic, while more traditional clans see symbionts as abominations and fleshcrafting as inherently evil and corrupting. Like everything in Eberron it's up to each DM to decide which side is right (if any).
In Warhammer 40,000, there are several examples of this form of artifice.
The Magos Biologis of the Adeptus Mechanicus also experiment in this field, as their teachings lead them to prefer bio-grafts and organic augmentations over the traditional cybernetics of their fellows.
Although it is often referred to as "genetical enhancement", the processus of using Gene-seed to create a Space Marine is effectively a form of fleshcrafting. Indeed, it uses various external genetically cultured biological augments to transform a lowly human into a powerful Adeptus Astartes, and Big.E made sure they would not be genetically transmissible as He didn't want to turn all of mankind into Astartes. Despite this, most works refer to it as "genhancement" to convey it is an (arguably) safe and controlled process instead of the product of mad science or some dark magic.
The forces of Chaos mutate the flesh and bone of their mortal followers as their see fit, whether intentionally by invoking directly into their power, or accidentally by being exposed to warp energies. The usefulness of these differ (as these can range anywhere from sprouting wings to allow flight, having a lash-whip tentacle for an arm that overloads the senses of your opponent, improved physical musculature that would give even Chapter Masters a run for their money, to just an unrecognizable lump of flesh somewhere on your body with no real benefit), but it takes a lot of physical and mental power to handle the mutations that the Dark gods bestow upon their followers, and also a lot of luck, as being too physically and/or mentally frail, or simply being at the bad end of their patron god's mood when they gain their attention, can result in them devolving into what we can only describe as the Dark God's failed arts-and-craft project.
The Tyranids' entire species is based on fleshcrafting, constantly absorbing new genetic material and analyzing it to provide new ways to modify and augment themselves on a physical and species level.
And, last but not least, let's not forget about Mr. Fabulous who's been
desecrating CAREFULLY PERFORMING SCIENTIFIC EXPERIMENTS UPON anyone or anything that had the misfortune PRIVILEGE to get in the same room as him for the last ten millenia in order to create 'the perfect human being'.
In Deadlands, "Patchwork Science", a primitive form of fleshcrafting that amounts to making Flesh Golems is mentioned in the second of its monster manuals, although Marshals are told emphatically that this is NOT to be allowed into player hands as an Arcane Background.
The Tzimisce from Vampire: The Masquerade have the Vicissitude discipline, which grants them the power to shape flesh and bone at low levels, do all sorts of weird stuff, and become a monster, a puddle of sapient Vitae, a fucking dragon, and, in case of one Metuselah, a virus (it's not clear, whether he became ebola or AIDS).
The Phyrexians from Magic: The Gathering are not a wasteful bunch. If you don't need a body part, and it can be used somewhere else, they will take it, and use it. They also have nanobots called glistening oil that turns creatures into phyrexians. MTG also has thrulls which are living creatures made out of corpses.
Fleshcrafter is a character class in FAPP that specializes in adding and removing assets and mutations to their self or others.
The Zerg from Starcraft are similar to the Tyranids, in culling essence from creatures to assimilate them, or alter one of their existing creatures with attributes from essence they have collected. No wonder Starcraft is accused of being a ripoff.