Imps: Used as labor, food, weapon practice, and any other nefarious thing their masters can imagine, these are the lowest denizens of the Infernal realms, aside from any captured people or creatures from Sarella. Imps are small, red, winged shapeshifters who can multiply quickly and require very little food.
Shrakes (aka Common Devils or Lesser Devils): Roughly human-sized, with red skin, a tail, and typically a black steel blade, these are one of the most common types of devil. Shrakes occupy the lowest rung of Infernal society that is still treated with a modicum of respect, even occasionally gaining command over their own imps or Shrake squadrons. They are akin to foot soldiers or commoners, having very little importance in the overall scheme of things but are very useful in numbers. Shrakes are notable for their versatility and teamwork in combat, typically behaving as a well-trained block of footsoldiers would. The big differences between Shrakes and regular foot soldiers are potentially hundreds of years of experience thanks to reincarnation, innate magical resistance, and equipment forged in the lava oceans of Infernia.
Spined Devils: These sturdy, flying creatures are covered in roughly foot-long razor-sharp spines. They are typically unarmed, instead preferring to stay out of reach and hurl their Infernal Bolts at their enemies. Spined Devils are considered a step above Lesser Devils and occupy a similar niche to traders or merchants, having some upward mobility but never seen in a ruling position.
Bone Devils: Few of these were seen during the Cataclysm, but were reportedly within leadership roles, commanding Shrakes, Imps, and Spines. Their entire body is made of bone, and they are reportedly fierce combatants.
Massives (classification as yet unknown): One of these was spotted in the Battle for Odin's Gate. Not much is known beyond the ability to toss fire and summon huge flaming axes. It dueled with King Gaius during the battle, eventually being banished back to its' home realm, though not before it yanked the Emperor down with it.
The Infernal realms are home to a surprisingly rich variety of devilish creatures. Not much is known about these realms, as no one has ventured into them and lived or stayed sane aside from Morrow's Shield, who refused to speak on the experience. It is known, however, that the Cataclysm made it far easier for them to pass into Sarella, and when the Doomstar was defeated it became very difficult for them to pass through once more. The devils still in Sarella when the Doomstar fell were stuck there, eventually all falling to combat over the following centuries, forcing their souls to return to Infernia to reform.
Artist's depiction of a Massive, based off of scattered eyewitness reports.