Potentates of Hell

The shinigami, or Potentates of Hell, are entities for which the neutral rings of Hell are their natural habitat. They are seemingly immune to the ravages of age or physical harm, but are neither undead nor incorporeal. Under the rule of Kelemvor, and arguably of Lord Arcus, the modest population of Potentates enjoy free rein to traverse the neutral realms of the underworld: the Incunabulum, Limbo, Purgatory and their preferred habitat, the Atrium.
The Atrium is a sprawling ruin from an endless city constructed by a civilization long lost to time, and certainly older than Oa itself. Loose rubble and blowing dust characterize the structure of the place, though the atmosphere is often more festive than the desolate, blasted environment would suggest. There is always music, wine and a horrifying, bone-cracking form of dancing on offer in the Atrium. Pebbles are kicked everywhere as the grotesquely extended limbs of these creatures snap in gut-wrenching crepitus to the incomprehensible beat.
As its listeners and artists are both long since exhausted of hearing every conceivable melody, shinigami music is excruciating to mortals. The sounds are either so advanced or so perverted from long misuse that they seem to be nearly random, atonal sounds bearing little resemblance to the tones of melodic instruments.

Interactions with Mortals

While most of the shinigami are useless gadabouts desperate for entertainment, there are a few individuals that seem to have been assigned (or have voluntarily undertaken) some form of supervisory powers over some or all of the Clerks of the Incunabulum. These shinigami, the ones properly called Potentates despite the broader usage of the term to refer to the species, are more likely to interface with emissaries from the Grand Mausoleum in righting wrongs pertaining to mortality and fate.
That said, even the most indulgent wastrel of the lot has the power and seemingly the right (or at least the leeway) to engage in commerce with mortals. The currency of that commerce is almost always an extension or reduction of that mortal's lifespan in exchange for something more tangible. The extent of the shinigami's power over the reality of mortals is completely uncertain, but it is known that they travel on occasion to the Prime Material Plane and the Infernal Plane seemingly without restraint, which suggests they may be more powerful than demons.
The Kindly Ones' lore suggests that there have been occasions upon which Potentates become enamoured or infuriated with certain mortals and make special exceptions for them upon their demise. That lore also suggests that shinigami that are destroyed on Oa simply re-emerge in the Atrium some time later. That lore also contains a secondhand report from a dubious source suggesting that they can meet a final death by disintegration and that this explains the omnipresent dust littering the ground in their home. That lore is poorly translated, but it seems to suggest some active acquiescence on the shinigami's part: they may be immortal until they choose to do something that causes them to die.

Language and Culture

Gender and Nomenclature

While the Shinigami seem to have a native language involving sharp inhalation and clicks of their teeth and mandible joints, they all seem to be fluent in Common (though in many cases they seem to speak in terms that are several centuries outdated).
The concept of gender is meaningless and distasteful to the Shinigami. While they speak Common, the Kindly Ones have noted in their records that they prefer "they" pronouns, but use exclusively male-derived titles and female-derived slander for themselves: Duke, Earl, Lord, skank, primadonna, hag, etc. (The Grand Mausoleum 's linguists and necropologists are absolutely obsessed with this phenomenon.)
Shinigami names rarely exceed one syllable. The Yishanim speculated this was because they have limited capacity to generate voiced speech, but the Kindly Ones, with the benefit of further learning about their extensive system of titles and honours, generally prefer the theory that in a small peer group of non-reproducing immortals the need for detailed identification is drastically attenuated.

Power Structures

Generations of scholars have noted that the Potentates are forever preoccupied with social status. They seem always to draw a sharp distinction between power, prestige and something they call "constituency", but the context in which the latter term is used has consistently baffled them. Shinigami always draw a distinction between a powerful Potentate, a Potentate with a prestigious title or accomplishment, and one with a "great" or "elite" or "meaningful" "constituency" -- but as their culture does not seem to entail any form of voting or collective action the phenomenon to which this term refers remains a mystery. There are multiple records of scholars and emissaries asking for clarification and being met with inscrutable analogies to other unfamiliar phenomena.

Interactions with Vega Spritzer

Bishop Spritzer had three interactions with the Potentates in her career. Twice she was ignored by the Clerks of the Incunabulum when presenting her petition, meaning that those clerks did not have adequate authority to welcome her. Her proper path in those circumstances was to proceed to the Atrium to determine which shinigami had claimed the right to deal with the petitioner, superseding the authority of the assigned clerk.
In both cases the petition was unsuccessful: once it was denied out of spite, and in one other case it was withdrawn when the cost demanded by the shinigami was too high. The third time Spritzer encountered the Potentates was during the Petition of Sylvio Overmere, in which they too stood down, even ceasing their music, to permit her access to the Lieutenants of Hell. This was an important development for Kindly Ones' scholarship, as the process of approaching the Lieutenants was not adequately detailed in Bishop Liepzig's recounting of his experience in 337WA. Many scholars were shocked to find that the shinigami were capable of such uniformity of action, and that the authority of the Lieutenants was seemingly so absolute.
In her private moments, Spritzer will admit to herself that although it was profoundly prestigious to have discourse with the Potentates of Hell, she preferred dealing with the Clerks of the Incunabulum. She always found debating rules and regulations an easier task than crafting a diplomatic strategy to persuade the unknowable minds of these eternal creatures. The lore of the Grand Mausoleum suggests that appealing to a shinigami's sense of novelty and curiosity is generally a winning strategy, with the exception of true Potentates who take an interest in the proper functioning of Hell. After all, if the bureaucracy falls into chaos, mortals could start infesting the Atrium and nothing could be more gauche.
On 19th Winterfall, 866WA, Spritzer made an unintended visit to the Incunabulum, where she met Reex, Second Lord of the Stygian Well and made a deal to cause a great underworld scandal at some point in the future in exchange for safe passage back to to the living world.