Fiends

Fiends are a type of Outsider which gains power through the consumption of Souls. This generally causes them to be violent, manipulative, hostile, or otherwise antagonistic, and certainly they are presented in this light, though this is a vague trend, not a rule. Plenty of Fiends are fine people, particularly when interacting with Not-Mortals.

The most famous of these Fiends are threefold; The Daemons, Demons, and Devils. Absolutely no confusion here. Sometimes they're respectively called Yugoloths, Abyssals, and Infernals, so they're a bit less similar-sounding.

Fiends are a form of Outsider, a subtype if you will, with it's own categories. But because it's not an exclusive type, it can be added to other things; Fiendish Undead, Fiendish Celestials, Fiendish Beasts, Infernal Machines, Abyssal Engines, etc. It's all allowed, but it's worth knowing which ones are Outsiders as well, which isn't all of them. There's a lot going on.

Many Fiends are described below;

Daemons
Daemons, or Yugoloths, are one of the most populous fiends in the universe. Most live in Hades and Gehenna, though movement across the river Styx allows them to be found in most of the "Lower Planes," exceptions being the Nine Hells and the Abyss, where they are NOT WELCOME due to the activity of the Blood War. They're even found on the banks of the River Oceanus, and indeed assist the Ferrymen in many cases on both planar rivers.

Daemons are interesting in that they have some elemental-like qualities, such as having an Ephemeral Form in addition to their Physical Form, analogous to the Elemental's Spiritual Form, and their being easy to bind to an object or creature. Daemons are also strange in that they have a relatively strong presence on the material plane, though this is often rationalized not as a planar matter, but a practical one; as far as fiends are concerned, they are more widespread and more populous than most others, so it only makes sense for them to be more populous on the material plane.

Demons
Demons, or Abyssals, are the most populous sort of fiend, but are much more prone to infighting; They come from the Abyss, and their nature as Demons rewards them for outright murder, not merely consuming souls. Murder not exclusive to mortals. This means that most demons are caught in an eternal battle amongst eachother, not outwardly-focused like Daemons and Devils.

Demons are technically winning the Blood War right now, being that they're the only major faction of Fiends to have made any progress on another fiendish plane; Both the first layer of the Nine Hells and several layers of Hades and Gehenna are home to a demonic host, where no organized force has ever touched down on the Abyss' Stygian Shores.

These Hordes are neverending, and many mages have claimed that they are also unstoppable. The best mages disagree, however, being that one of those famous mages, Mordenkainen, removed all demons from a layer of the Abyss. These mages simply point out that the efforts would NOT be worth it... Though, none of them have offered a solution for the Heart of the Abyss.

Devils
Devils, or Infernals, are by far the most organized of the Fiends, and come from the Nine Hells. These are the ones that give Fiends a manipulative and dishonest reputation. Depending on where and which ones you're talking about, they might behave like the mafia, like lawyers and business associates, or like nobility and peasants, but the relationships are almost never equal. Devils gain the power of those less than they are, such that if every devil below one devil attempted to overthrow the top devil through power, they will fail outright.

Devils are the current "biggest loser" of the Blood War right now, though in times past they were much more successful. One problem is that, unlike most Fiends, Devils are almost exclusively made through corruption, as opposed to normal reproduction methods. It means that their legions are less full than the others, particularly compared to the Blood War participants.

An infernal Legion is unbreakable, their formation is perfect and their morale is absolute; They fear their higher ups more than they could fear any enemy they could fight.

Succubi
Succubi are nice. They're Fiends with connections to lust and boning and whatnot, but despite their perpetual horniness, they're usually pretty fine people. They have obvious natural talents as Charmcasters and similar roles, but there are also many succubi with few to no abilities related to charm and the mind. They usually have the ability to enter the Ethereal Plane whenever they wish, and as such they make wonderful assassins, among other things.

Succubi are also interesting because of their lust; It's not just being horny(Though, they always are) but a matter of survival. They CAN survive on other things, but they actually do need to bone to be healthy. Many, because of this necessity, learn to use it as a means of generating magical energies, and other such resources. On the upside, it's like getting spell slots from eating food. On the downside, this makes them even more reliant on sex.

They have a place in the Blood War; Many places, indeed. They often ally themselves with other Fiends, such as the above Daemons, Demons, and Devils, and as such get roped into the war in every conceivable position.