A Balrog of Morgoth?
“Living to at least 30 metres inside caves, it makes webs to catch flies, but mostly preys on woodlice and millipedes on the cave walls. “
I first read this and thought the spiders were 30 meters long and live in caves. Frankly a 30 meter long spider can live where’ve the heck it pleases.
And the cave spiders are Shelob’s children.