Bebiliths are huge predatory, arachnid demons that hunt other demons. While their favorite prey is Tanar'ri, they aren't picky - they will stalk and attack any type of creature. They have the ability to shift into the Astral Plane and their attacks can rip armor to shreds.
They are actually natives of the Astral Plane, formed from the genocidal hatred of the wars between the Tanar'ri and obyriths.
Sources:
PS Monstrous Compendium I
City of the Spider Queen
Monster Manual (D&D 3.0)
A Guide to the Astral Plane
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