The Legion of Gold is the largest, oldest, alliance of Outsiders in the Lower Planes, predating the creation of the Tanar'ri and the foundation of Baator. Indeed, it is the largest, oldest alliance in the entire multiverse. The Laws of Heaven were only ideas, and mortals mere beasts, when the Legion was founded. The members of the Legion call themselves 'Soldiers of Gold'.
The word for that epithet in Old Infernal is Yugoloth.
The Yugoloths are not a species.
They are a polity, an army, a disparate, self-cannibalising, disloyal, destructive, heaving legion, constructed of the myriad castoff Fiendish kinds created by those ancient avatars of true evil, the Baernaloths. Even the evil of the Abyss pales compared to their foul campaign across creation, waged since magic was young.
Among their numbers they count many loyal strains of fiends - the leadercaste called 'Ultroloth', or 'Masterful Soldier', or the low-ranking thugs referred to as the 'Dhergoloth', or 'Hungry Soldiers', for example.
These species have their own names, but they are unknown to mortals, perhaps even to their fellows in the Legion.
Some, less loyal, who are neither Tanar'ri nor Baatezu, drift closer to their orbit - the Oni of Acheron, the Concubi of the Abyss, the Ravaasta of Hades (who are called 'Arcanoloths' by some).
Every kind of fiend - exiled Tanar'ri, hideous Vaporighu, defecting Baatezu, bizarre Maelephants, secretive Gehreleths, scheming Night Hags, surviving Obyriths and even the supposedly independent Rakshasa, can count representatives within the Legion - and enemies without it.
What of the Legion's leadership? Certainly, the Ultroloths hold authority, and they hold above all their lord, the General of Gehenna. Supposedly, when the General sits upon the Siege Perilous, (his throne of petrified, living demons) which lies atop Khin-Oin, the Godbone Tower, the End of All Things will begin.
Supposedly.
In his absence, the High Oinoloth, a master plague-crafter, is scheming chancellor of the great tower.
The Legion's founders, the Baernaloths, embody a form of utterly apathetic, sociopathic evil - a kind of almost banal, low-level, omnidirectional hate, without passion.
They saw everything but themselves as an object or a tool, to be taken, used, broken, and discarded. They do not lead the Legion - indeed, most fear it, as almost all of the ancient Baernaloths met their end at the blades and claws of their spawn.
It was this first rebellion that inspired the Baernaloths to create the Books of Keeping, ancient tomes, now lost, which contained in their four-hundred and forty four volumes the names of every member of the Legion.
Most were burnt by the vengeful 'Loths, and, by now, the Legion's numbers far exceed such paltry records.
The potential of the Books as a weapon against the Legion is a strange and baseless rumour - some say spread by the Yugoloths themselves, to decieve their adversaries.
What of their laws? They hold none, not any more, none but the Law of Gold, the reason and excuse for the Legion's mercenary actions across the Multiverse.
None judge a Canoloth for guarding a vault in the City of Doors. None judge an Arcanaloth that illicitly advises a Bytopian Sphinx on matters of magic. None judge an Ultroloth for leading an army across some Prime Plane. None would even judge a Dhergoloth for taking a contract to knit.
So long as the Law of Gold is not broached - 'Nothing Costs Nothing'.
The Legion think in expenditures and profits, an attitude beaten into them by millions of years of commerce, slavery and mercenerial wars.
There are no mercenaries their equal in all creation. They act with professionalism, ruthlessness and skill (if not charm or class), as long as their payment persists. Mortals refer to the Primeval Pacts of Baator and the loose agreements of the Abyss as 'Contracts', but the Legion are the only fiendish organisation with a specialist caste of contract lawyers (The Yagnoloths).
They are not the teeming horde of the Abyss, nor the faceless myrmidons of Hell. They are the elite. They use all the most modern technology - the average Ultroloth wields a jezail armed with silver shot, alongside the traditional longsword.
The conflict to which they devote most of their time is the eternal Blood War, a conflict which many accuse them of both sparking and prolonging, for their benefit.
To the average mortal, for whom greed and violence are common, the Legion are at once the most understandable, and the most reprehensible of all the Fiends.