There are many things to beware of when dealing with a devil, the three most important being:
Fairness.
The devil does not play fair. He deceives and uses honeyed words to swing situations in his favor. He will use any underhanded tactic available and is ready to invent new ones immediately.
Deception.
The devil is far from devilish. He dresses with grace, more elegantly than angels or royalty. He is punctual and speaks curtly, with airs of nobility and grandeur.
Exchange.
Most importantly, the devil does not offer fair exchange. He offers an opportunity of fortune in your direst hour in exchange for what's most precious to you. The choice is always yours to make, however, sometimes there is no other choice.
There are two ways of making a deal with a devil. The first is to summon one using a ritual. The greater the sacrifice, the greater chance for a devil, especially one of greater status and power to be summoned. Importantly, this method of summoning guarantees a deal with the summoned devil.
The second way is to simply encounter one roaming amongst the populace. Despite their elusive nature as shapeshifters, they tend to dwell in the mortal realm to gain experience and enjoy themselves. You may request a deal from the devil if you so encounter one, and the devil can similarly offer you a deal in this scenario.
Deals with the devil cannot be broken under any circumstance by either party. It is the inborn divine authority of the devil race. Punishments for breaching the contract must be specified within the contract and may vary greatly; there must always be a punishment.
If you wish to hunt down a devil, despite the challenge, there are many important things. Devils are birthed from an abundance of malevolent aligned energy found plentifully in hell, but can also spawn from high concentration areas in mortal realms. The amount of malevolent energy used to birth a devil is usually an accurate indicator of its future potential.
Malevolent energy is created from an abundance of death energy, strong negative emotions, cursed items or the death of another devil. By absorbing this malevolent energy, devils can progress in power. However, a better way for a devil to grow in strength is through devouring souls which directly enhance their power and potential.
Devils come in all shapes, sizes and forms. Most are humanoid, donning varying outer qualities such as tails, horns and wings; this though is not a principal as some have been sighted as animal like creatures of more fearsome anomalistic growths.
Their power, despite their varying appearances can be categorized as such:
Hatchling stage: Devils that have just been birthed. They are like toddlers and cannot speak, cast magic or coordinate their body. They are stronger than the average human toddler but are for the most part harmless. It is best to eliminate them in the hatchling stage so they do not grow stronger.
Lesser imp stage: Devils that have learned to walk and use their naturally high strength (given they are humanoid). On average, they are still inept spellcasters, however regular human soldiers should still be weary. It is also at the latter parts of this stage that the devil learns to shapeshift, a skill that becomes greater at increasingly higher stages.
Greater imp stage: These devils grow much bigger than their infantile self, almost as big as a young teenage human. They are able to utilize most functions of their body (wings, horns, etc) and can cast entry level dark magic and curses of which they specialize in. Importantly, though devils are almost all naturally attuned to the evil categories of magic, there have been anomalies.
Lesser fiend stage: These devils usually reach the maximum potential of their physical size and have formed their devil heart. Due to this, their capacity for spellcasting and general combat ability increases drastically.
Greater fiend stage: Only after absorbing a certain number of souls can a devil reach the greater fiend stage. After becoming a greater fiend, a devil's natural-born talent is awakened granting them a significant increase in power. It is recommended for most combatants below the fourth tier to avoid them as devils are scheming and tricky to fight.
Entry demon stage: After entering the demon stage, devils gain greater enhancements to their natural born talent in addition to greater battle capacity accompanying every stage increase. Most demons are able to seamlessly use evil category magics and arts. Entry stage demons are also able to resist lower level holy magic.
Mid stage demon: also known as the demon soldier stage, mid stage demons are able to take down small baronies in few numbers. Other than higher holy magic resistance, concrete information is difficult to pinpoint as demons of this level are usually quite unique in their powers.
High stage demon: also known as demon generals, these demons are able to singlehandedly take down baronies overnight and can go toe to toe with viscount or count level territories. Boasting even greater resistance to holy magic, they usually require manpower from Ducal houses to take down. Demon generals are, however, extremely rare in the mortal plane with only a handful appearing every decade and only in large scale invasions.
Arch demon stage: Only a handful of arch demons have ever appeared on the mortal realm. These demons are usually the leaders of large invasions and can pose a great threat even to the imperial forces. They are immune to all but high tier holy spells.
Demon king stage: No demon kings have ever appeared on the mortal realm. It is only through interrogation and memory extraction that their existence is even known. There is no definite number of demon kings nor are their power levels clearly defined. It is suspected that the appearance of a demon king may spell the end for a kingdom.
And within the abandoned mines of a small Barony–the city of Gull–a demon god was to be born. One who so happens to enjoy the torment of others very much.