http://www.d20reviews.com/Daemonforg.../fiendlord.htm
The Lesser Lords
“There are lesser Lord of the Broken Lands if such a title is appropriate for beings of such power. Many did not spring forth from the depths of hell but are powerful beings such as dragons and titans who have fallen under the sway of hell. The most infamous of these beings is The One Traitor of the elder wyrms of the World”
As you see all fiendlords are not necessarily fiends...
So you can create a fiendish giant society, no prob.
A good place to put them is in the Border (since its between the Patrons and the major Fiendlords. Here´s also the realms of Ilvataris the Dragon "Fiendlord"
The Border – Land Of The Broken
Geography
The axis of the world is Mount Barakus. It is the point from which all of creation came to be. The lands surrounding it were once the most prosperous and blessed of all the nations of the World. People from the South, the North, the West and the East came to found nations here. Among its forests and fields, rivers and lakes, rolling hills and misty swamps, they all prospered.
Now it is a blasted and shattered land. Here the last and most savage of all battles were fought between the fiends and the mortals. The land itself was wrecked and despoiled by the magic of the combatants. This was over one and a half thousand years ago. A thousand years later the land was ravaged anew as the wrath of the fiercest patron fell upon the land, and five Lords were slain by his hands. Now anarchy reigns in the territory as fiend turns against fiend seeking to replace the slain Lords.
Leonid Kozienko
Population
The Broken is how all the mortals are known across all the Broken Lands, but above all it describes the peoples of the Border. Unlike the nations of the West, the North and the South who were laid to waste by the fiends, the nations of the Border survived in a twisted sense. As the mists descended the senseless slaughter of the conquered mortals did not cease, but it did abate as the fiends came to value the conquered as a valuable resource. Many mortal realms and territories bowed to fiendish masters, pledging their loyalty in exchange for mercy. Thus the fiends came to be as nobles and the mortals their serfs, slaves bound to the land. Ever they suffer, either at the hands of their dark nobility or by the actions of the highland patrons whose armies seek to incorporate the borderlands under their rule. Looked upon with contempt by both the conquerors and the unconquered.
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So the giant realm could be new or old...