The Old Gods were first. They existed before historical records were kept, and if the myths are to be believed they were the ones who spun the world and the universe itself into existence. They made mortals as self-directed toys and playthings; our self-determination and free will were a side-effect of their efforts, not the intent.
Each of the five Old Gods is its own unique monstrosity, reveling in some concept or combination of concepts and taking them far beyond the realms of mortal sanity. They are inherently alien, maddeningly far beyond mortal conception of what is possible or real. Each was, and still is, a God equal to the True Gods; only the fact that the Old Gods wounded one another in the final years of the Godswar allowed the True Gods to steal fragments of their divinity and lock them away within their Hells beyond the Veil.
Even in their prisons, they still have ways to communicate with and grant power to those few of their servants who remain on Mundus; fortunately, though, for the most part they and the Demonic Princes and lesser Demons who serve them are trapped and unable to directly affect the material plane. That all changes when the Veil thins and the Churning begins, however, and it is times such as these when the lore of the Old Gods’ evils becomes a matter of life and death for the mortal peoples of the world.
The cultists might be working to end the world as we know it, but at least they're still mortal; the demons who serve the Old Gods are something other, fiends ripped out of mortals’ nightmares or the eldritch dreams of the Old Gods themselves brought to life. Each of the Old Gods has a variety of demons serving them, varying in power level from the pitiful Rustwretches of Dis to the reality-breaking Aggrothic Muses of Pandemonium.
Whatever their level of power, demons are fundamentally inhuman; they have different needs and minds from mortals, sometimes radically so. While some are expert at understanding how mortals think, none of them have an ounce of empathy or sympathy for mortals who are not useful to the demon or their masters.
Most demons created or employed by the Old Gods bear the mark of their masters upon them. Demons of Ferukhar are feral and savage meldings of mortal, beast, insect, and plant. Demons of Amekhar are experts at corrupting and exploiting mortals, and are driven by a burning desire for a wide variety of things. Demons of Radokhar are twisted psychic vampires, able to amplify mortals’ emotions and feed upon them. Demons of Helikhar are ordered, purified creatures, with an otherworldly beauty that hides their brutal philosophy. The demons of Zunakhar are twisted abominations, either the products of eldritch experiments or intent on subjecting other creatures to such horrors.
Above all of these creatures stand the Demon Princes, demigods born and shaped in their masters’ twisted images and placed at the head of the Old Gods’ armies and servants. There are five Princes for each Old God, an unsettling mirror to the five Saints of each of the True Gods. Each Prince is unique, but each and every one has the power to match a Saint should they ever come to blows. They are just as trapped in the Hells as their Old God masters, but their Heralds can be sent through to Mundus during a Churning, and can be terrifying forces on or off the battlefield.