Fiends

The denizens of the Lower Planes are as diverse and storied as the mortal races of the Material Plane, but all of them can be categorized under the umbrella term of "fiend," a moniker they have quite readily earned thanks to their inherently dark and insidious dispositions.
Like other fully extraplanar beings, fiends' physical forms are malleable, serving more as a vessel for their otherworldly essence as opposed to a mortal body that both contains and encompasses a mortal soul. For a fiend, their outward appearance is a reflection of their spirit's strength, with more powerful fiends being able to conjure more powerful avatars. For most fiends, they increase this strength by defeating their fellows in combat. When a fiend kills a fellow on their native plane, the felled fiends essence is absorbed by the victor, adding to its own power. Overtime, this allows even the lowliest fiend to grow and evolve. Indeed, the archdevils and demon princes that rule the many layers of the Lower Planes are merely the product of the end of that cycle and represent the pinnacle of strength a fiend can achieve.
However, most mortals will only ever interact with a fiend that has been brought into the Material Plane, either summoned there by a mage or arrived of its own volition through a Planar rift. On the Material Plane, a fiend must take on a corporeal form in order to move about and interact with its environment. Moreover, it must tend to its flesh as a mortal would; needing to eat, breathe, and poo in order to survive for any meaningful length of time. Unlike a mortal though, a fiend "killed" on the Material is merely returned to its home plane, as its essence is inexorably tied there. Because of this, the only way to permanently destroy a fiend is to seek it out on its home plane and destroy it there.

Demons vs Devils

While there are countless sub-types of fiends, most can be broadly categorized into two distinct categories: devils and demons. Devils chiefly reside within the Lower Planes known as the Nine Hells and can be distinguished by there "civility" as opposed to their demonic cousins. Only a fool would consider devils to be any less dangerous than demons though. Most scholars will tell you that devils are the far-greater evil. Their intelligence and cunning allows for far greater malice and evil to be wrought. Devils will put on an enticing venire and ensnare mortal souls with under-handed promises and two-faced contracts. At least with demons, you can always rely on them to just try to kill you.
Speaking of demons, these are the fiends that reside in the Abyss, the lowest of the Lower Planes. These are monsters born of rot, death, and decay. They are often generalized as ravenous, unthinking brutes that wish only to kill everything in their paths, and this is admittedly the case for a vast majority of them. However, the upper echelons of demon-kind boast a cold, malevolent wit that is just as dangerous as their fangs and claws. However, where a devil will attempt to beguile its victims with double-speak and catch them on technicalities in their contracts, a demon will simply lie, cheat, and betray its way to its goals. Those that are willing and skilled enough to make dealings with fiends all follow a similar credo: with enough insight, intuition, and precaution, a devil can be dealt with. A demon can never be trusted under any circumstances.

The Blood Wars

Fortunately for the Material Plane, the bloodthirsty residents of the Lower Planes are far too engaged with killing each other to attempt any major attack on the realms of mortals. Devils and Demons have been participating in an eternal war at the border between the Hells and the Abyss since time immemorial, and there are no signs of this perpetual back and forth resolving any time soon. Ultimately, each side wishes to gain the strength of the other, but the devils are too inundated with backroom scheming and betrayal to fully push back the demon hordes, who in turn are far too unorganized and beast-like to ever overtake the regimented legions of the devils.

Loumara Demons

Loumara's are ironically an exceedingly rare exception to the general rules of demons, despite being the source of most demonic stereotypes on the Material Plane.
When considering demons, most mortals think of incorporeal shadows lurking in the darkness; tormenting poor peasant families, causing supernatural phenomena, and ultimately possessing their victims and pushing them to murder their families and loved ones. But as it turns out, very few demons actually behave this way. A quasit may stalk its victims and instill crippling fear in them before killing and devouring them, but they still have a physical form with which they carry out these tasks.
Loumaras, however, have no such physical form. They exist as living shadows in their homes on the Abyss. Even on the Material Plane, they are fully incorporeal beings that require a host to survive long term. Despite their rarity, an encounter with a Loumara always leaves a lasting impression, thanks in no small part to the difficulty in dealing with them. They have no physical form to slay, and once they have latched onto a victim, it takes powerful magic to exorcise them. Simply put, an infestation of lemures is easily dealt with by a small band of fighters and quickly forgotten about, but a Loumara haunting is something that sticks in the memory of its victims for a lifetime (provided there are even any survivors left to tell the story).