Demons, otherwise known as Hellspawn, Demonkind, and the Minions of Darkness are evil entities native to the Burning Hells, and are the primary enemies of the Diablo series. They come in a variety of shapes and sizes and can wield a wide range of magical powers depending on their type. They propagate and serve to spread pure chaos, making them diametrically opposed to the angels of the High Heavens (who stand for Order and fight for the Light). The never-ending war between angels and demons is known as the Eternal Conflict.
"Who can say what drives a demon?"
Demon physiology differs greatly. Thick hides and claws may be possessed by demons, but that aside, demons of the same dimensions may still have dramatically different weaknesses, depending on their master. Megademons are considered to be archetypal demons. Some demons possess wings.
Demons, unlike angels, are physical beings, and their bodies remain after death. The souls of deceased demons are always reborn in Hell. Demons are apparently spawned from the Black Abyss, though examples exist of breeds of demon being created through artificial means. Demons do not age, or at least not in a manner recognisable to mortals.
Demons are wont to breed with each other. They have demonstrated the ability to crossbreed with angels (the results being nephalem), and with humans, usually to the horror of the latter. Few children of such couplings survive childbirth, and fewer still survive into adulthood, as they are generally killed by those who know them for what they are. Deckard Cain also suspected that Barbed Lurkers (a type of Ancient Beast) are partially demonic as well. Encounters with demons by humans can result in psychological scarring that lasts potentially indefinitely, though measures can be taken to minimize harm.
At least some demons can possess humans and masquerade as angels. To best knowledge, demonic possession of a human cannot be ended without killing the host in the process. There are measures that can be taken which can ward off demonic possession, though some practices make humans more vulnerable to possession (e.g. practicing dark magic).
Demons do not need to sleep.
At least some demons possess blood. Dry demon's blood left on armor can react adversely to water, causing the armor to be destroyed in a flash of green light, after generating sulphur/blood-scented smoke. At least some radiate arcane energy. Most greater demons possess lots of horns.
Demons are cowardly, and will attack using strength of numbers to their advantage whenever possible. They are prone to "unnatural" levels of distrust, even towards their own kind. Many demons are vain to the point of self-denial.
Wounds suffered from demons' blades don't close easily (at least wounds inflicted against humans).
Worship and Summoning
"Ye ain't gettin' me to believe in demons. Stories, that's all them legends are. Just meant to give a man a laugh an' maybe a sense of unease now an' again."
Many humans do not believe in the existence of demons, though stories of demons became common in folklore of some human cultures. Some philosophical writings within the East have postulated that demons are nothing more than a representation of Man's nature (i.e. "his demons"). Yet demons do indeed exist, and some mortals worship(ed) demons, and have engaged in summoning practices. Examples include the Triune (a font for the Prime Evils as benevolent entities during the Sin War, the Coven (remnants of the Triune, turned to the worship of Belial) and the worship of Kabraxis in an attempt to eclipse the Zakarum faith. The Zakarum faith itself was corrupted by Mephisto. Mortals have also summoned demons to the mortal realm, a practice which dates as far back as the Vizjerei Clan. While such a practice has since been banned by the clan, groups such as the Coven continue it. Centuries past, when summoning demons was common, it was common to invoke lesser demons and subdue them in order to gain their power. Many sorcerers ended up being enslaved to their intended servants. Possession by a demon may grant a human increased power, but this comes at the cost of one's sense of self and eventually, a demon can completely overtake the human's form with its own.
If summoning a demon, one is not reccomended to do so during the months of Vasan and Lycanum, as during this time, demons are erratic and easy to anger. In contrast, the most ideal times to summon demons is the period of time between the last days of Jerharan and the first days of Damhar. Furthermore, if summoning a demon, one should never attempt to do so alone. Rather, the summoning should be conducted in a group consisting of three, four, or seven individuals (representing the Prime Evils, Lesser Evils, and combination of the two respectively).
While not necessarily summoning or possession, at least some demons within Hell can speak through humans on the mortal plane.
In ages past, it was common in Sanctuary to decorate buildings with stone statues of demons that were enchanted to serve as guardians and protectors of those within. The misshapen figures of the statues were designed to taunt and anger any demons that attempted to gain entrance.
Beliefs and Culture
"I shudder to think what the demon lords might accomplish should they ever stop betraying one another. Such is the self-defeating nature of evil."
Demons believe that absolute chaos is the true nature of Creation. Many lesser demons despise humanity, seeing the existence of Man as offensive to their "higher" position in the greater scheme of things. Many demons hoard wealth. Attitudes of demons otherwise depend on their master—the legions of Mephisto were driven by a hatred of all living things, especially those who were pure of heart. The soldiers of Baal sought the undoing of Creation, striving for disorder and destruction. To them, order was an abhorrence. The minions of Diablo used illusion and fear as their greatest weapons, preferring to wait for an opponent to show weakness before they attacked. Demons frequently scheme against one another, but otherwise, a hierarchy of sorts can be observed for demonkind:
- The Prime Evils—Diablo, Mephisto, and Baal.
- The Lesser Evils—Azmodan, Belial, Duriel, and Andariel.
- The Mallet Lords (roam the outer reaches of Hell, do not answer to the Great Evils)
- The Balrogs (often allowed contact with the Prime Evils, oversee lesser demons)
- Counselors (often rise to positions of power within Hell, some only answered to Balrogs and the Prime Evils)
Of these Evils, many believe that if there can be said to be one single ruler of demonkind, it is Mephisto. At the very bottom of the hierarchy are the Fallen, who are despised by all of demonkind.
The Primes and Lessers (collectively known as the Great Evils) each had their own realm of Hell, while the Mallet Lords control the outer reaches of Hell. It was rare for the Primes to work together. Few demons are allowed the honor of direct contact with one of the Lords of Hell. Many servants of the Great Evils exist(ed), such as the Sin Lieutenants of Azmodan.
Demons maintain a symbol-based writing system. Few, if any humans are capable of reading it.
Many demons fear the light of the sun.
Demons have a sense of "justice" (such as it is), but it is a cruel and sadistic form of it. Demons can be notoriously clever, and should not be trusted by mortals. However, they hide their lies within the truth.
"Once you see the kind of things I've seen, you'll be a pacifist too. Demons don't count."
— The Memory of Caldessan(src)
Demons originated from the corpse of Tathamet, whose body became the Burning Hells. For ages they fought against angelkind in the Eternal Conflict. It came to a halt with the disappearance of the Worldstone. It was eventually revealed that the Worldstone had been used to create the mortal realm, a revelation that gained the interest of the Prime Evils. The result was the Sin War, which ended with a pact between Heaven and Hell to allow Man the freedom to choose his own path.
The Dark Exile and Civil War
"How dare you question! How dare you deny yourselves victory! Long have the Primes lorded over us. Long have they thought themselves our betters. Diablo...Diablo will fall, as will the rest. Do not be swayed or led to fear by him. Do not, for each of you will reign. Each of you will share the power we will earn."
— Belial addressing Duriel and Andariel prior to the coup(src)
The aftermath of the Sin War led to a falling out between the Prime and Lesser Evils. Not only had the Lesser Evils been kept in the dark about the existence of Sanctuary, but they were furious that the Primes were intent on weaponizing humanity's nephalem potential rather than continuing the Eternal Conflict against Heaven. As the centuries plodded along, many in Hell began to wonder just what it was they were fighting over. The Lesser Evils of Hell were growing tired of the endless ebb and flow in the tides of mortal favor, as well as those in the elaborate schemes of the Primes. The Lesser Evils longed to return to the epic days of the Great Conflict, and lay siege again to the gates of the High Heavens. In their ignorance, the Lesser Evils began to believe that the Three were afraid to continue the war with Heaven. Believing themselves superior to mortalkind, and frustrated by the cessation of the war, Azmodan and Belial saw the situation as their chance to overthrow the Prime Evils and take control of Hell for themselves. The two demon lords made a pact with their minor brethren, assuring them that the wretched plague of humanity would not deter the ultimate victory of the sons of Hell. Azmodan and Belial devised a plan to end the stalemate, achieve victory in the Sin War and ultimately ride the bloody crest of the Great Conflict straight into the very arms of Armageddon. Thus, a great revolution was set into motion as all of Hell went to war against the Three Brothers. The Primes, despite their efforts, were defeated and banished into the mortal realm, bodiless, where Azmodan hoped they would remain trapped forever. Furthermore, with the presence of the Primes in the mortal realm, Azmodan believed that Heaven would be forced to divert its attention towards humanity, thus potentially leaving Heaven vulnerable.
The few demons who still pledged allegiance to the Primes entered Sanctuary to serve their masters. Yet relations between Azmodan and Belial quickly soured as they began arguing over who should ascend to the highest rank in Hell. When the bickering turned to violence, their alliance ended, and the remaining denizens of Hell found themselves in a second civil war. This state of affairs continued until at least the Darkening of Tristram, by which point Azmodan had gained the upperhand for quite awhile. However, the two demons nonetheless came to share the same goal of subjugating humanity into their fold.
The Reawakening of the Three
"And the Evil that was once vanquished shall rise anew. Wrapped in the guise of man, shall he walk amongst the innocent, and Terror shall consume they that dwell upon the Earth. The skies shall rain fire, and the seas will become as blood. The righteous shall fall before the wicked, and all of Creation shall tremble before the burning standards of Hell!"
The awakening of Diablo in Tristram sparked a chain of events that upset the status quo for demonkind. Andariel and Duriel concluded that the Primes would inevitably use the powers of Man to reclaim Hell, and thus endevoured to gain favor by allying themselves with the Lord of Terror as he traveled across Sanctuary with a mortal host. It was believed that with Andariel's presence on Sanctuary, the forces of Hell were once again aligned with the Primes. Diablo, Baal, and Mephisto gathered in Travincal, where they enacted their final stratagem to retake Hell and gain vengeance against those who overthrew them. Opening the Infernal Gate, Diablo entered Hell to rally demons still loyal to the Primes. Baal journeyed north to corrupt the Worldstone, which would corrupt humanity in turn and enslave Man to the will of Hell. Mephisto stayed behind, intent on destroying any who would attempt to thwart their plan. In truth, the Three had masterminded the Dark Exile and their banishment with the aid of the fallen angel Izual, who had told them of the soulstones and how they might use them to further their own ends.
Yet the Primes' plans were thwarted by a group of heroes. The group that had slain Andariel and Duriel, Mephisto, and Diablo in Hell itself. Baal himself was slain on Mount Arreat, though not before the Worldstone was corrupted. To save humanity, the archangel Tyrael destroyed the stone. While this saved humanity from corruption, it left Sanctuary open to demonic invasion.
Dawn of Damnation
With the destruction of the Worldstone, demons were able to travel east further than they'd done so in the past, coming into conflict with the Priests of Rathma. Five years after the destruction of the Worldstone, Skarn rallied a demonic army in order to obtain its fragments, intending to resurect Diablo and reignite the Eternal Conflict. Skarn's demonic forces launched an invasion of Sanctuary, sweeping over Westmarch. Soon, only the capital stood against them.
The End of Days
"We are lost. Diablo is too powerful. Always before, the Lords of Hell bickered and worked against each other. They undid themselves. Within Diablo, within the Prime, there is no such division, and now the halls of the High Heavens quake beneath his steps."
Belial and Azmodan came to share the same goal of subjugating humanity into their fold. Belial sank his claws into Caldeum, ruling Kehjistan through his mortal host, yet his plans were thwarted by the Nephalem and his soul imprisoned inside the Black Soulstone.  In a bid to retrieve the stone, Azmodan unleashed his army upon Sanctuary and laid siege to Bastion's Keep, yet was also defeated and imprisoned. Yet it was here that Diablo's plan was revealed as the artifact was turned on the mortal Leah, bred as a vessel for the Lord of Terror by her own mother, Adria, for his rebirth as a singular Prime Evil. Now, as a singular Prime Evil, Diablo led the legions of Hell in an assault on Heaven. For the first time in the history of the Eternal Conflict, the Diamond Gates were breached, and demons ran rampant throughout the Silver City. Yet once again, Diablo was defeated, and the demon invasion thwarted.
In the aftermath of Diablo's defeat in Heaven, and that of Belial and Azmodan on Sanctuary, lesser demons still wandered Sanctuary, thirsting for the blood of the innocent. However, the loss of their leaders meant that they acted without larger organization, and did not make much of an impact (at least in the eyes of Heaven). That said, decades after Diablo's defeat, demons still roamed Sanctuary, continuing to roam the land unchecked. Demonic invasion reduced Kehjistan to ruins, and after the empire's fall, demons continued to roam the desert sands.
In all three Diablo games, demons are one of the three (the others being undead and animals) major types of enemies the heroes will encounter, and perhaps the most numerous category. Within the user interface, a demon is usually associated with the Life indicator.
Demons are more or less encountered in nearly every location of Diablo III. Bonus effects against Demons are less common than in the second game, but still exist: for example, Tyrael's Might. The only act that has less demons than other enemies is Act V.
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