Bebilith

Called barbed horrors or creepers of the Abyss, bebiliths are huge, misshapen spiders in thick chitin. Each front leg ends in a heavy hooked barb, and venom drips from a mouth crowded with fangs. They communicate only with each other, by mind contact.

A bebilith attacks anything it sees. It is never surprised, ignores nonmagical weapons and anything below +3, and is always cloaked in protection from good, which it can reverse at will.

Each foreleg deals 1d6 and can wreck gear: roll 1d6 per hit, 1-2 for the shield, 3-6 for the armor. Mundane gear is destroyed on 40%, minus 10% per magical plus. Ruined gear gives no AC, costs its full gp value to fix, and loses any enchantment permanently. Unarmored targets just take normal damage.

The bite deals 1d12 plus venom (save vs. poison at -2 or die in 1d4 rounds). A poisoned corpse not blessed within a turn bursts into flame and disintegrates.

Four times a day it can spin a permanent web, otherwise like the web spell, filling 8,000 cubic feet in whatever shape it likes (such as a 20-foot cube), starting beside it and reaching up to 60 feet. Fire has only a 25% chance each round to burn through.

Hard pressed, it can plane shift at will into the Astral, hauling along one foe it is fighting who fails a save vs. wand. A victim able to leave the Astral alone can still go.

Bebiliths hunt, or perhaps punish, the tanar'ri, choosing particular bands of greater tanar'ri by unknown means and wiping them out in short, ghastly wars. What such chaotic beings do to earn this is a mystery.

Though many Abyssal powers could kill one easily, none do: slaying a bebilith is taboo. Travelers have fled into one's territory to shake off tanar'ri pursuers, then had to escape the spider. Illusory bebiliths work only sometimes, since demons seem to know the real thing by smell and perhaps spiritual aura too.

That aura theory comes from witnesses who describe creeping despair near a bebilith, though plain terror would explain it just as well.

What is known was bought with many lives. Mages and alchemists pay 2,000 gp and up for a spinneret, convinced without proof that it powers binding spells and items.