The Arachno Corvus was once a massive Underdark mega-fauna, acting as a apex-predator in the undedarks largest caves. It was some time ago driven mostly extinct by the sapient races of the Underdark.
It is believed by some Drow Scholars that some might have survived in hidden corners of the world, but are of a much smaller size.
The Arachno-Corvus resembled a massive gargantuan spider with a head resembling that of a crow or raven. It could climb most surfaces and its beak could burrow trough most types of rock. It is believed that the creature was created in ancient times by either Lolth or The Raven Queen, but neither gods or their followers are willing to take credit.
Today their massive skeletons dot the caverns of the Underdark, some are used in the construction of Underdark dwellings, while others remain where they first fell serving as points of interest or way markers.