Fiends
It is easy to look at the various fiends and lump them all together as a blob of confusion and violence but this would be a mistake. There is value in understanding what they are, what they are trying to accomplish, and how they intend to do that. Their motivations are their own and they can be used to predict how they might respond (or in the case of the chaotics, expectations that cannot be relied upon). Knowledge is power and the knowledge of one's enemy offers the power to avoid an early demise.
Devils
Not everything that exists in the Nine Hells is a devil, but their reluctance to share power on any level ensures that for the most part devils are the only thing LEFT in the Hells. Hellwasps and Nightmares are permitted because they are useful in some way or another. Who knows what other beasts were native to the burning gyre but were exterminated for not showing a similar value to the Ba'atezu?
The structure of civilization in the gyre has three tiers, like a pyramid of power with the Ba'at at the top, the Reforged beneath them, and the innumerable Damned beneath. The Damned are farmed and mined for particularly useful individuals to reforge and they are tantalized by the promised elevation of the Reforged and the limitless power that can be attained if only the will to succeed is harnessed and the weakness purged. All of this is of course a lie, as the path to success does not go to the top. Reforged are rewarded for their drive and cunning but they are set up to fail and that failure throws them back down the path. The Reforged will never stand as equals of the Ba'at. The Nine claim a pure meritocracy but the fact is that the only Reforged to have been elevated to Ba'at are Ba'at who have been demoted previously. The long lifespans of the Ba'at and the short lifespans of the Reforged ensure that this situation will remain unchallenged.
Wicked and cunning souls are drawn to the Hells by whatever unknowable currents pull at all mortals. Most never make it to the shores of the Styx, tested and purged on their way and relegated to oblivion, a momentary snack for those that shepherd them. Those with the will to survive are expelled from the Styx as Lemures, their physical forms only as solid as their fragile will can manage. Those who resist the pain of their existence by walling themselves off become Nupperibo, desensitized and unforgeable by the Ba'atezu and so doomed to wither away to nothing unless they embrace their pain and return to Lemure state. Those with a will to harden but unable or unwilling to conform and submit precipitate out into Infernal metal to be forged into useful objects for the Blood War. Those with both will and cunning can be reforged as lowly Imps, the first step toward power in the Hells.
The greatest of the Ba'atezu are the Ba'at, immortal celestials cast down into the fires or willing volunteers to stand against the consuming chaos (depending on who you ask). Unlike the Damned and the Reforged, they cannot truly die even in the heart of the gyre but their destruction surely comes with a performance review and commensurate loss of station. Many Imps will tell you that they are the legendary Eternal Imp, a Ba'at who offended Asmodeus so greatly that he was reforged to the lowest order as a warning to others who would dare challenge the First Among Equals. Surely most of them are lying, but one of them might not be.
