While 3e makes little mention of it - and 4e outright raped them - the very first ultroloth created the artifact known as the Heart of Darkness, which he then used to purge his race of the impurities of law and chaos. The impurities found their way into the larvae of the Gray Waste, who the yugoloths then herded into Baator and the Abyss. The yugoloths themselves were created by the baernaloths, rumored to be the primordial aspects of Evil - they're one of the great secrets of the Lower Planes.
However, given the recent revelations behind the baatezu and tanar'ri, it's likely the larvae tainted by law and chaos became the Ancient Baatorians and the obyriths, who were then displaced by the baatezu and tanar'ri.
They are the architects of the Blood War. It's a giant experiment in the nature of Evil, and they'll end it on their terms. Meanwhile, they keep the other fiends jerked around like puppets while they hurl lesser yugoloths into the War like disposable trash. To an extent, they are; unlike the baatezu and tanar'ri, when a yugoloth dies a new mezzoloth rises from the Waste, Gehenna and maybe Carceri as well. They sneer at gods - the children of mortal faith - and they fact that they aren't made from mortal souls perhaps elevates them higher than the common devil and demon. While both pit fiends and balors look demonic and tower over their fellow fiends; the ultroloths are worse - they're tall humanoids garbed in black or gray cloaks and robes, and they're faceless save for a pair of fiery eyes. All ultroloths look alike; the paragons of faceless, remote, uncaring Evil.
The three yugoloth towers - the Tower Arcane of Gehenna, the Khin-Oin of Hades and the Tower of Incarnate Pain in Carceri - are actually part of a larger scheme of the loths; one that will make them the undisputable lords of the Lower Planes. Unfortunately, the Tower of Incarnate Pain is still under construction, and the gehreleths make constant raids on it at the behest of their god, Apomps.
The lords of the yugoloths are the ultroloths; there are none above that rank. The General of Gehenna was the very first, and the greatest. The Oinoloth is the ruler of Khin-Oin; the former Oinoloth was an altraloth called Anthraxus the Decayed, who was ousted from that post when an ultroloth prince called Mydianchlarus whispered a secret or lie so disturbing it shattered the Oinoloth's sense of identity. Now he wanders the planes; seeking powers and deals to reclaim his former throne; but even the gods fear what he may do and what he may yet become.
The fact that many of the yugoloth lords have disease-based names (Mydianchlarus, Anthraxus, Diptherius, Typhus) is perhaps rather telling; the yugoloths are like viruses, and they've infected and infested the Lower Planes like a malign, living disease.
It should be noted that the baatezu and tanar'ri were, at one point, stripped of their ability to greater teleport by the 'loths, although they've since restored it.
3E kind of really glosses over the loths, though; to the point where the ultroloths are still stuck with their 13 HD instead of being brought up to par with the pit fiends and balors.
I'm reconverting them to 3.5e in the "Everything Else RPG" subforum of this site.
