The Unholy Legions of the burning Lower Planes are too numerous to count! There seems to be a rough ranking amongst them seemingly based on influence or raw power of some kind. Within these broad bands they may vary greatly in appearance, goals and abilities but all are roughly equivalent. The differences in personality appear most markedly from the Major Demons and upwards, with distinct individuals being known to many mortals throughout the ages. The unaging aspect of these entities along with their ability to bind souls leads some to hypothesise they are akin to the Gods in that they may increase or decrease in rank and power over time, waxing and waning according to their influence. Perhaps even the mightiest like Sorban or Sethis were once little more than grovelling Imps...though it is not recommended to voice such theories loudly!
Infernal beings are often found as manipulators behind the actions of warlords and tyrants across all cultures, as servants of more powerful Demons or summoned in the service of powerful (or foolish) Warmages. Needless to say this tends to anger any nearby Clerics greatly! Warmages find their Demonic allies less likely to rebel the closer their alignments are to one another and may glean important information from ones they encounter if they ever learn their Demonic language...
Demon Lords and their lessers are often resistant, if not completely immune, to elemental Fire damage but display a great vulnerability to Cold and some weakness to Holy damage. The higher ranks are unique in their foibles and may or may not share such traits...
The Infernal Ranks
Imps are mean, but feeble and often bound into service. One even served as scribe for Drakenwood in ages past!
Daemons are more capable and cunning, as shown by the capricious Network Daemon of Covah Island!
Demons are commonly encountered and cause much tragedy, such as the Drakenwood Lighthouse situation...
Major Demons including Takaklar, often bound and found in the servitude of more powerful beings.
Greater Demons such as the cunning Baal or dread Shuggothea.
Demon Lords comparable to the Soulcrusher the Sentinel or the horrific Guanthanothua.
Demi-Gods akin to the evil Spider Queen of the Drow, Lloth.