Atropals are undead abominations that were said to be originally meant to be immortals modeled after the gods. However, since this work obviously went unfinished, they became what they are now - ugly baby things (or Ghouls on steroids in 4E) that seek to bring death and destruction to the planes. They can only speak gibberish, but don't be fooled - they are actually highly intelligent, fitting of what they were meant to be. Most of them are found adrift in the Astral Plane, the Elemental Chaos, and the Negative Energy Plane, though some are either with another being of considerable necromantic power (such as the one in the Tomb of the Nine Gods, being nourished by Acererak's Soulmonger) or sealed away in forgotten places and objects.
Whatever the case, its a CR 13 (in 5E) Large Undead that can pose a serious threat to a party singlehandedly, especially at lower levels, being durable enough to tank many strong hits and having powerful abilities relating to its nature as an abomination of undeath.
While the idea of them being unfinished godlike beings works, there are hints that there is a more sinister reason for their existence. It's been repeatedly hinted in multiple settings that Atropals share some connection to a certain World Born Dead beyond similar names - most notably, them sharing a resemblance to non-Aspect avatars of Atropus. Thus several theories alternative to the mainstream "failed attempt at godlikes" arose in-universe around this connection.
One such theory is that Atropus is simply the first, largest, and most powerful Atropal . The mainstream theory could still hold if that is the case, though why one Atropal is fucking moon-sized would have to be explained. It does match the part though - its surface is partially composed of amniotic fluid, which is associated with childbirth (and thus the ugly baby motif of most Atropal renditions) and it only seeks to bring death and destruction to the worlds of the Material Plane.
However, other theories place Atropus as the creator of this race of undead abominations. Instead of being created after the gods and left unfinished, they were created by the Elder Evil itself in its image, probably as a means of creating powerful emissaries of its will to spread death. This was either actively done, possibly with its Aspect shaping each Atropal from the moonlet's flesh and soil, or passively by Atropals being formed by material being cast off of its surface by impacts and the like.
There's also the unreleated theory of Atropals just being miscarried or aborted gods. Though its not as intriguing as the others, it does make sense, considering both in-universe and in IRL mythology there's plenty of god-babymaking happening.
The most powerful Atropals can attain some semblance of their original purpose by becoming Atropal Scions (no relation to our boys in blue). Becoming even uglier, highly grotesque, and decayed fetus-looking things (Nurgle would be proud), these things are considered "godlings" (whatever that is) and are, as expected, even more intelligent and powerful than their normal kin. It's implied the Atropal Acererak was keeping around would eventually become one of these if the Soulmonger kept nourishing it. They can also be formed if one were to take a piece of a god and make it undead. If summoned to the material plane, these abominations plot in the shadows, biding their time and gathering power before attempting to wrest control of its newfound world. And if that succeeds, the entire planet can be expected to gradually turn undead (or abruptly, as a successful Atropal Scion takeover often attracts the aforementioned Moonlet).
In addition to the strength and intelligence boost, Atropal Scions become powerful spellcasters even by the standards set by the rest of their kind, as well as gaining an aura of cold (dealing 3d10 cold and 1d10 necrotic damage (the latter also being subtracted from their maximum health) to creatures that start their turn within 15 feet of the Scion) and a stare capable of killing (said stare being treated as a DC 20 Constitution saving throw, against a flood of negative energy caused by its stare, that instantly kills and zombifies on failure, and turns corpses directly into zombies - though it can only do the former once a day). If Atropals were a serious threat to a party, these things are party-wipers if not handled carefully. Their challenge rating accordingly raises to be around 25-26, depending on the spells it has.