There are any number of Great Old Ones slumbering in Crypt 
and indeed in all of the worlds, each with its own "theme" 
just like a normal pantheon (mad god of war, pestilence, death,
etc.). All of them pay homage to Nameless One.

The Nameless One headed a pantheon of Great Old Ones.
All of them are distant, unknowable beings of mostly formlessness 
and varying degrees of malevolence. The Great Old Gods, at best,
barely notice Gifted Ones, and at worst, openly feed on them without
rhyme or reason. It is believed that the borders of the Retroverse 
were defined by the spread of the Elder Races and their Nameless 
God (for to say his name is to drive the speaker mad). 

The Great Old Ones were defeated by the Other Gods, the 
current pantheon that rules much of the Retroverse.  Sikkar 
takes claim for imprisoning the Nameless One in particular, but 
this is debatable.  There are some who say the Creator imprisoned
the Nameless One, others who lay claim to a variety of deities or 
all of them in concert for its defeat.  Some believe that the Nameless 
One has always slumbered, and that the Other Gods stumbled upon
their slumbering tombs, only to create the Elder Sign. Nevertheless,
all sources agree on two points: The Nameless One slumbers, and 
the Nameless One will awaken again.

Unlike the Nameless One, the Great Old Ones CAN be awoken, 
and will go about recruiting followers to bring about the Nameless
One's release. The Nameless One's pantheon differs in that they 
are alien beings who have existed for millions, perhaps billions
of years. 

They are alien, bizarre, usually not holding to the normal 
humanoid form. Their magics are strange, and they had 
superior technology (that functions on a level of magic) that is 
lost even to them, although they seek it wherever they can. 
Demons and hell, on the other hand, coexists in a layer-like 
fashion with the physical plane, as heaven does -- but the Elder 
Sign extends throughout all three dimensions, trapping angels
and demons and devils alike in it.

So even in Heaven and Hell, there are boundaries and the Gods
are trapped. The Nameless One and his pantheon are not part of
this good/evil dichotomy at all. They are destroyers and murderers,
but they are not evil -- they are beyond evil. Evil's the word mortals
put on what the Nameless One's worshippers do to keep themselves
from going insane.