Fleshbones

Flesh to Stone's reversed counterpart, Stone to Flesh, is well known for being the cure to a Medusa attack. It's a very intelligent spell. It can look at the structures present in stone and infer that it used to be a creature, recreating that creature and all its organs, even preserving the mind and soul. This all seems to be mostly taken for granted but it's very impressive when you think about it.

Of course, any apprentice learning this spell begins to wonder about the effects of casting Stone to Flesh on actual stone - rocks that were never alive to begin with, inanimate objects and castle walls. The results are almost universally unpleasant as the spell attempts to reconstruct tissue, organs, and other functional structures where there were none to begin with. If it manages to create something alive then it won't live long, though there are some notable exceptions such as the Necrolith and Mortimer's Throbbing Castle.

The more material you give the spell to work with, the more it can achieve. On the flip side, the spell can also make good use of high levels of detail. A few talented gastronomancers have carved beautifully marbled steaks out of actual marble. Some particularly foolish mages have attempted to use the spell on carved statues to create an artificial human; the results of that are far worse than "unpleasant".

Bone really muddies the waters (don't get me started on Stone to Mud) when it comes to being made of stone or flesh, which I'm sure you can imagine is very confusing if you are a spell and your goal is to turn one into the other. Bones are made up of stuff you can find in rocks and crystals but they have a cellular structure like living flesh. Sometimes there's even a little bit of residual soul-stuff. And yet, there isn't the macro-structure a spell would usually study to recreate a functioning organism - no sinews, no muscles, no blood vessels, no nerves, no organs.

Flesh to Stone has the expected effect on skeletal remains: it turns them to stone. Stone to Flesh usually fails, but under the right circumstances - when all other organic matter is decomposed or removed and the bones are dry - the spell can mistake bone for stone. If a detectable amount of soul essence lingers, the spell might even take that as a sign that this was once a living organism that turned to stone. And to be fair, it isn't entirely wrong.

The way it tries to "reconstruct" this creature, however, is absolutely wrong.

Not finding any structures from which to reform muscles and organs, the spell can sometimes set to work on creating its own. The spell cannot add any matter, only convert what is there. The end result is a monstrosity that is superficially similar to the common necromantic skeleton found in tombs, in that it walks mindlessly and attacks intruders - yet this skeleton is entirely made of soft flesh, coated with skin and filled with blood as you'd expect from any fleshy creature. It can barely hold up its own weight.

The residual soul essence occupies the new body, so for all intents and purposes this is a true undead like any wight or ghoul.

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AI-generated, I am not an artist.

Fleshbones

Initiative: +0

Attack: punch +0 melee (1d3) or by weapon +0 melee

AC: 8; HD: 2d4 (5 HP);

Movement: 20'; Action Dice: 1d20;

Special: undead, half damage from blunt weapons, horrific, confused biology

Alignment: Chaotic

Saves: Fort +0, Ref +0, Will +0;

Horrific: When injured, the gory reveal of the monster's composition causes psychological trauma to anyone who was not expecting the bones to be fleshy. Bring your own sanity system, here's a good one.

Confused Biology: When killed, there is a 50% chance it is cut/broken where it is hit and the separate parts survive independently due to unusual organ arrangements (treat as two new Fleshbones). Once these parts are killed, they stay dead. All broken parts have 5' speed, except a complete pair of legs which has 30'. d4: (1) decapitated, (2) arm severed, (3) legs separated from torso, (4) leg severed.