Infernus. Pandemonium. The Infernal Dimensions. Human demonologists seem so confused. These are all the same place. I have been there. It is a magnificent place, a world more like Freneskae than Gielinor, made of jagged rocks and liquid fire. We call the denizens of this world demons, but they are actually many peoples and their history goes back millennia or longer. The earliest race - at least, that I know of - were the Infernals, and though I do not think they were the original rulers of their planet, they were at their height the undisputed masters of the demon races. Thousands of years ago, the Chthonian race rose up, overthrew the Infernals, and took their place as overlords. The other major race were the Avernic, who had been slaves under the Infernals and remained slaves to the Chthonians.
Despite their chaotic and highly varied appearance, the Chthonians were creatures of laws and rules, and their culture was a fussy bureaucracy that had largely replaced open warfare with a legal system. They were cannibals, and could steal the power of any creature they consumed. By contrast, the Avernic were still tribal and primitive, and constantly engaged in bloody squabbles. There were many Avernic subspecies, the largest and strongest being the Tsutsaroth demons, sometimes called Elders or Butchers. A unique quirk of the Avernic meant that the Chthonians were unable to consume them. The Tsutsaroth ruled over the smaller, winged Alyaroth and the yet smaller, wingless Byzroth with brutal authority.
At some point prior to his arrival on Gielinor, Zaros visited their world and tricked the Chthonian ruler into giving him control of twelve legions. Though he was never master of Infernus itself, even just a few legions gave Zaros military power unmatched by any of the other gods. The legions were formed of Chthonian nobles leading hordes of Avernic shock troops. This relationship continued in the Zarosian empire, but the vast human population elevated both castes of demons such that even the Avernic were among the elite of society. As the Chthonians lived lives of distant luxury, they failed to notice that some urban Avernic were beginning to develop a sentiment of rebellion. Zamorak played on this to sway many Avernic to his side, although only two of the Tsutsaroth were prepared to make an outright pledge of allegiance.
After the destruction of Zaros, Zamorak fulfilled his pledge to the Avernic by travelling with them to Infernus to overthrow the Chthonians. Even with his new divine powers, this took him nearly twenty years, but by the end every Chthonian had been destroyed or exiled to the Abyss. The Avernic became fiercely loyal to Zamorak as a result, and would serve as the backbone of his army during the God Wars. Over time, the Infernals and Chthonians were forgotten, and most humans know of only the Avernic. The Chthonian remnants still linger in the Abyss, however, and I know of at least one attempt to summon them in the facility at Viggora's Folly. Bilrach had always spoken of seeking out the lost Infernals, but I do not think any of them can still exist after so long.