Atropus

Atropus, known as the World Born Dead, was an alien entity whose coming was a harbinger of death and destruction. According to some, it was in fact an undead primordial.

Description
Atropus was both a place and a being: an undead the size of a planetoid, whose surface was rocky and barren, but nonetheless inhabited by all manner of horrific undead. It had the seeming of a massive chunk of black stone, pitted and slick with putrid black slime. From certain angles, Atropus resembled a hairless head with glassy eyes approximately across, the moonlet itself measuring approximately  in diameter.

Atropus' aspect, or focus, was a headless, vaguely humanoid-shaped creature with fossilized flesh coated in the same putrid slime as Atropus itself.

History
Some sages speculated that Atropus was created when Ao created the first gods, though they didn't have a unified theory about how it was born. Some claimed Atropus was the rotting amniotic fluid of the first god of death, while others believed he was a divine mistake by Ao—the last god formed, that arrived dead upon creation. Others stated that he was really one of the primordials.

Atropus was first observed for the first time in the ring of the planet Glyth. It was attracted to the planet by the widespread death and destruction the mind flayers wrought on the other beings of their world. Atropus laid waste to the planet, leaving it dead and devastated. Some of the few sages who knew about Atropus believed that its attention was turning toward Faerûn on Toril next, slowly venturing over to the life-rich planet. They were ever vigilant for signs of its approach—a strengthening of necromancy and the spontaneous rise of the undead.