Children of Entropy was written specifically to provide a "Foes" type manual, but with each group of NPCs being created around a theme -- For Example...
Crocodilian Mallia Cultists
Desert Vampire Vivamort Cultists
...In order to provide scenario hooks.
It was also written to ensure the NPCs were neither utterly unbeatable (although some are as hard as a coffin nail) nor feeble (one of the problems with "Foes").