Devils

Devils are Fiends, typically formed from the corrupt souls of wicked mortals, who sustain their existence through rigid structures of rules and hierarchy. As a result, they are dedicated to the philosophical principles of order and law as embodied in statute and writ, and to the absolute terms of any pact or agreement into which they may enter. They grow their own power by constructing a pyramid of names, each of which is anchored by the terms of the pacts they have sworn to in that name. The pacts of devils are ironclad, because making pacts is not something that they do, but rather something that they are.
As part of their pursuit of order, devils seek to impose their will and vision on the cosmos. As a result, they constantly seek to retain and accumulate power. The more power they have, the greater the pacts they can forge and thus the more power they can gain. In addition to personal power, a devil's status is part of a great, infernal hierarchy, and devils gain additional power from their place in this pecking order, both from their dominion over others and their fealty to their superiors.
The diabolical drive for conquest and control is diametrically oposed to the purely destructive nature of the Demons of the Pits. Faced with this nihilistic aggression, the Devils muster beneath the greatest of their number to defend the Pillars of Aiaos and preserve the world that they wish to rule. To this end, each Pillar is ringed by one of the Nine Hells, a fortress-city formed by anchoring the power of an Archdevil to the pillar, and where infernal warriors muster for battle and their masters plot their own ascensions.
Devils deal in two currencies: Contracts and souls. Contracts give them leverage to control the material world and potential access to more souls, but also define a devil's being and create its status. Souls, meanwhile, are the fuel of the Abyss, as well as potential recruits for the infernal legions. Demons never deal in anything as ephemeral as 'favours'; all infernal contracts are explicit in their terms and their penalties, and no devil will ever give more than a contract stipulates. They will take more, if it is offered, but if offered without a contract, will never give anything in return.

Pacts, Names and Hierarchy

Devils exist by the law, as embodied in their pacts and contracts. These pacts are sealed by the name of the devil, and to break the pact is to break that name. For a devil to breach a contract is to undo a part of their own being, and it is a devil's names which give them power and identity. The greater the devil, the more names it has, or more accurately, the more names it has, the greater the devil, and it is by the sealing of pacts and the acquisition of names that a devil ascends in power. New names are acquired through accumulating contracts against the names that a devil already holds, and greater names enable the devil to forge more powerful and efficacious contracts.
Diabolical hierarchy is based on the number and type of names they hold. Only true, diabolic names and titles count in this system, and any devil may have any number of lesser aliases.

The Nameless

Nameless devils are nearly-shapeless, lacking any definition of form or personality. They are without intelligence or language, merely obedient to the commands of greater devils. Lemures are essentially a larval state in which unharmonious souls emerge from the Styx, but the bloated nupperibo are those who are unable to gain or maintain a name regardless of their time in the Abyss. These hollow creatures are sustained only by the authority of those that command them.
In order to ascend to a higher level of existence, a nameless devil must be granted a name. This name is usually granted to them by a higher devil, but can be given by any being, so long as it is done deliberately and with some degree of ceremony (at the bare minimum, a statement of the form 'I shall call you ...' and the granting of a meaningful token will suffice.)

Singular Devils

Singular devils are those with only one name. Such names are usually short and unimpressive, and not infrequently insulting. They are also bound up in a physical token, the loss of which strips the devil of their name. A higher devil will almost never seal a bargain against their singular name, first because it is considered undignified, but more importantly because it is the foundation of their identity and breaking such a pact would destroy all of the devil's names.
The most common singular devils are imps, but spined devils and hell hounds are also singular. The form that a singular devil takes is shaped by the name and token that they are given. The most powerful among them tend to be unambitious shock troops, while imps, although relatively weak, invariably feel driven to increase their power.
A singular devil seeking advancement will eagerly seal pacts against their name - although they are unable to deliver anything but their own direct services - as they need to secure and fulfil nine such pacts to forge a name of their own choosing and become ascendent.

Ascendent Devils

Ascendent devils possess two and only two names; their original, singular name and a second of their own choosing, which they forge on completion of their ninth pact. Their singular name remains bound to their token, and after their ascension they will keep it secret. Their chosen name is the one against which they will seal pacts. This name is more impressive and holds greater power, and so the pacts forged upon it can offer minor supernatural boons to those who accept them, as well as the service of the devil and possibly some of its cohorts.
For the most part, ascendent devils are soldiers and enforcers of varying power, including bearded, barbed, bone, ice, scroll and chain devils, nergaliids and merregons, but if they can seal and fulfil nine times nine pacts against their greater name, they can advance to the rank of paramount. Given the time it might take to contact and secure pacts with mortals eighty-one times, if not with eighty-one separate mortals, ascendent devils often supplement this number in two ways: by undertaking to engage in services contracted by greater devils, and by acting as an agent to secure pacts for singular and other ascended devils.

Devils Paramount

Devils paramount possess between three and six names, binding additional names to themselves as they progress through the ascendent ranks. They are leaders among the infernal, and can offer significant powers and gifts to those who would deal with them, as well as the service of lesser devils.
Such devils, including rakshasa, abishai, narzugons, eaters of hope and orthon, either devote themselves to maintaining and advancing in the hierarchy of the Hells, or choose to make the forging and binding of pacts the work of their immortal existence. Those who choose the hierarchy undertake services contracted by their noble overlords, their goal to achieve high favour and be granted the chance to fulfil nine great pacts - pacts which change the shape of the world in some momentous fashion, which may include a warlock's pact, if the warlock achieves great things - and achieve their own noble name. Those who become bargainers, meanwhile, drive hard towards the nine times nine further pacts which will allow them to create an unbound name.

Unbound Devils

Unbound devils, including abhorrent overlords, horned devils, nightmare shepherds and erinyes, possess five or more names which they forge through their bargaining. Their highest name is always their unbound name, which is the name secured by nine times nine paramount pacts, and which ensures them some measure of independence from the rigid hierarchy in which other devils exist.
The unbound name is a name which can seal pacts as great as those of the noble devils, but unbound devils do not risk this name lightly. Only a pact of great power and moment will tempt them to swear on their unbound name, the kind of bargain that can forge a name of nobility.

Noble Devils

Noble devils - primarily the amnizu and pit fiends - possess more than seven names, including a name of nobility founded on nine great pacts.
Noble devils are able to forge still greater pacts, and do so with greater impunity than the unbound. While an unbound devil risks demotion by dealing on their unbound name, the noble name is sealed into the same token as the devil's singular name, and if a noble pact is broken the devil can surrender another of their names in its place. Noble devils may also acquire one, or more, unbound names and use those for sealing major pacts.
At this point, a devil has reached almost the pinnacle of their career. A noble devil who wishes to go further has only two choices. They can seize control of a Hell, make it their domain and bind their name of nobility into that domain so that they can become an Archdevil, or they can secure a sufficient quantity of pure, divine essence to attempt apotheosis as a god.