Righteous Clay

Yes, that is totally a Flumph PC.

A few early issues of Dragon magazine featured a special insert called the "Creature Catalog" (which is different from a  later hard-backed book, also called the Creature Catalog, which was an early version of AD&D's Monstrous Manual). Most of the creatures from these inserts have vanished from the game, though a few such as the Hamadryad, the Burbur and the Orpsu have made a few random obscure appearances in later editions.

Creature Catalog III from Dragon #101 features a monster called a Righteous Clay, created by Howard Granok and originally illustrated by Marsha Kauth.  A Righteous Clay is, at its name implies, a sentient lump of gray clay that skulks around caverns waiting to suck the souls out of anyone who wanders across it.

Wait, what?

Yeah, that's right.  It's an evil ball of clay that devours souls using an ability named, appropriately enough, Soul Gouge.

 So what exactly is a "soul" in terms of D&D? In later games, particularly the Planescape setting-- which allows your characters to explore the various Heavens, Hells, Astral, Ethereal and Elemental Planes-- the soul is your personality and "essence" which passes on to another plane of existence that fits with your alignment (i.e. you go to whatever Heaven your culture believes in if you're good, or your particular brand of Hell if you're evil).  It's what makes you you.

Thus, when the righteous clay steals your soul, your body is left an empty husk.  Not a dead.  Just empty.  You breathe and sleep. You move if someone pushes you. You eat if someone puts food in your mouth. But there's nothing inside you anymore.

And what does the clay do with your soul, exactly? It... gains a few hit points.  Your soul is food. That's it.  It uses the very essence of a sentient being the same way you'd use a cheese sandwich.

And what's even worse is, according to the text: "righteous clays are so named because of their extreme arrogance and self-centeredness."  It KNOWS what it's doing to you and your loved ones.  It's just too self-absorbed and snooty to give a crap.  It's not even like the clay has a vastly alien mentality and just can't conceive of its prey as a sentient being.  It can understand Common and can actually converse by vibrating its body, " in the manner of a stereo speaker" to quote the text.  They don't care about the pain and loss they're inflicting. They're just hungry, dammit.

It is possible to recover the souls a righteous clay has stolen if the beast is killed, then a Reincarnation, Remove Curse, Restoration, Resurrection or Wish spell is cast on the empty body the soul was taken from. So some of the horror is blunted. Especially in a high magic world like, say, the Forgotten Realms, where you can pop down to Main Street and pay a mage to give you a spell scroll.  

But what if the Clay escapes and vanishes into the miles of black caverns that it inhabits?  Or what if you're a poor farmer who can't afford magic? Or a goblin or kobold whose beneath the notice of other beings (hey, goblins and kobolds have loved ones too!). Then your soul, or the soul of a close friend or loved one, is gone forever.

I'm actually surprised this monster never made a return appearance anywhere.  It's absolutely terrifying.

Even though righteous clays seem a lot like oozes, slimes and puddings, I like to think they're a completely separate creature entirely. To me, the various ooze-monsters are giant monstrous slime molds like myxomycetes, dictyostelids, labyrinthulomycota, or some other sort of protist/fungi. Righteous clays, on the other hand, are literally sentient clay-- a soil material formed from extremely fine particles of feldspar (minerals containing aluminum and silica) with small traces of oxidized metals and organic matter. Though "alive", they don't have any cellular structures.

In our world, some scientists theorize that the development of life may have been aided by clays because they can attract and protect organic molecules and facilitate polymerization (the development of long chains of carbon-based molecules like carbohydrates and fats).  Maybe in a fantasy world, some of this early "life" remained within the clay, giving it a bizarre animation of its own.

Relating to this idea, many Earth mythologies describe a god creating human beings out of clay.  Maybe righteous clays are scraps from that original sculpting process.  Or perhaps rejected prototypes that have retained some of the living essence that the gods originally imbued into the soil.  

Yet their form of "life" is imperfect.  They constantly crave that which the gods gave their more fortunate relatives-- an eternal soul.  But the clays are only partially developed, and thus lack the ability to retain the souls they steal.  Instead, they metabolize them to power their own torturous existence, quickly destroying that which they hunger for most. Perhaps their arrogance and selfishness comes from envy of the god's more perfect creations. A defense mechanism to avoid facing what they are. 

Oh, also,  elves and half-orcs can be affected by the Soul Gouge power, even though, to quote the text "(they) do not possess a soul per se". That just raises so many questions.  Why didn't early D&D elves and half-orcs (and presumably orcs) have souls?  Where did they go when they died? What was animating their bodies and giving them personalities?  And, if they don't have souls, what is the righteous clay feeding on when it attacks them?

You confuse me sometimes, Old School D&D.