An enormous being the size of a subway train, created to build the giant cities of the Old Ones. Just seeing it is a mind-blowing thing. The name means "Inhabitant of the kingdom of darkness."
Currentaly living in vast lightless caverns and tunnnels under Antartica.
It was a terrible, indescribable thing vaster than any subway train - a shapeless congeries of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and un-forming as pustules of greenish light all over the tunnel-filling front that bore down upon us, crushing the frantic penguins and slithering over the glistening floor that it and its kind had swept so evilly free of all litter... "At The Mountains of Madness" by H. P. Lovecraft