Bebiliths (GAP)

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Most sources name the Abyss as the cause of these terrible creatures. What very, very few realize is that the Astral Plane is really their home. The adventurous Indep sage Kalo Seth was the first to notice that when these creatures were in danger (which doesn't happen often) they fled to the Astral Plane. He noticed that through some natural ability, they can travel to the Astral whenever they wish, sometimes taking prey with them against the poor sod's will.
What Kalo suggests is that the bebiliths are actually creatures of thought, born on the Astral like an astral searcher. Bebiliths are different, however, because they consist only of penitence and guilt. 
These fiendlike horrors punish evil with evil, yet somehow manage to earn evil's respect. Kalo suggests that they are only just starting with the tanar'ri, methodically destroying them for the wrongs they've committed. From there, they will move on to other evildoers, perhaps cosmically ending with themselves, creatures of evil in their own right, for the sins they themselves are guilty of committing while carrying out their task.
This is all conjecture from one man. No one knows if he's right or wrong, and the bebiliths sure aren't talking. His theory does explain why the tanar'ri believe that harming the bebiliths is taboo, despite the fact that they are methodically hunting them down and destroying the fiends. If they are creatures of evil's own subconscious guilt, then the Astral would definitely be the place of their birth.
Kalo suggests that the bebiliths travel to other planes by a form of reverse astral projection. That is to say, they are creatures of pure thought that magically create physical bodies for themselves and send them to physical planes. This would mean that some silver cordlike link exists between bebeliths on the Abyss and their "real" mental bodies on the Astral. It also means that destroying the physical form of a bebilith on the Abyss does not permanently destroy the creature by any means.
Kalo said that he found an ancient citadel that predates virtually any known mortal civilization, built out of the dismembered pieces of a dead god. This citadel, constructed of memories and dreams made solid, was the home of the creatures of living penitence. It was the citadel of the bebiliths, a catacomb of astral webbing (apparently the true astral form of the bebilith has some mental counterpart to the physical bebilith's spiderlike webs) and stonelike god flesh. No one else has ever been able to find it, and Kalo refuses to go back. He likewise resists any effort to make him tell more of what went on in this rueful warren of punishment, other than to say that the screams that came from the place alone guard it from intruders.
Bebiliths are abhorrent, misshapen spiders with chitinous shells. They possess barbs on their two forelegs, and the fangs in their mouths drip poisonous fluid. If a bebilith hits with one of its forelegs, it will cause 2d4 points of damage. Their bite not only causes 2d6 points of damage, but the victim must also make a successful saving throw versus poison with a -2 penalty or die in 1d4 rounds. For further information, reference the bebilith in the PLANESCAPE MONSTROUS COMPENDIUM Appendix.