A very large amount of creatures of all shapes and sizes have appeared in and around Hyrule throughout the series. Ranging from gigantic dragons and undead stalfos to the diminutive miniblins, they come in all types. Any adventurer can easily agree that Hyrule has a very diverse collection of monsters that have troubled the people and challenged heroes. However, there are a great many creatures that have appeared and have merely been called demons. What they really are or where they come from is rarely elaborated upon. The only common element anyone can agree on is that they appear to be a very malicious group of supernatural creatures with a desire to dominate and destroy. They are even hinted at being another race if not another classification of being entirely. Sometimes monstrous and sometimes perfectly human, they are definitely creatures of wonder that have taken a huge part in the history of Hyrule.
The demons appear to have a higher status among the monsters in Hyrule and have sometimes appeared in the mythology of the story. They can at times appear to be more supernatural and otherworldly than the typical foes, such as ordinary animals or bandits. Other times they are not only a threat to the hylians but the gods as well; usually the greater threats or more pure evil creatures are labeled as demons. This matches their description in real world theology as well since the concept of the demon is the oldest and purest form of evil. Often Zelda’s demons are led by a Dai Maou or the Great Demon King, which is a title many of the main antagonists have held at one point in the past. Starting with Ganon in his earliest appearance, the title Demon King has gone on to describe later antagonists such as Malladus, Bellum, and Demise. Given that so many different characters have held the title in the past, it might be more of an honorary or earned title.
Since the demons in the series have appeared in all shapes and sizes, they are very undefined overall. They are typically humanoid in shape and can either be physical or like spirits. Often they have a few monstrous features, such as horns or wings like those of Chancellor Cole or Batreaux. Many can take a more humanoid form that totally hides their nature like Ghirahim does. And many of them have shown to be capable of taking a variety of shapes at will, which makes classifying them even harder. It is also possible for mortals to be transformed into demons through various means, as shown by characters like Ganon or Vaati. Typically they are not as easily defeated as the standard monster and require certain special implements or items that allow Link to defeat them; usually the Master Sword or Four Sword fills this role. The majority of the time there is no origin given for them whatsoever. They are simply another aspect of the fantastic realm that the series takes place in. Only in the backstory of Skyward Sword are they said to have risen from the cracks of the earth to attack the people of the surface.
The sword is now imbued with the mythical power to drive back demons, and only Link may wield it!”
Often these demons cannot be killed despite being defeated by Link. Even in the legends told as the backstories of various games, the major demonic entity is not something that could simply be killed, even by the greater powers of the world before the modern hylians came to live in it. They were often sealed via some form of magic. Whether by the power of a goddess, a sword, or the Spirit Tracks, the more powerful demons had to be imprisoned since killing them completely was simply not possible. Another notable detail is that demons, including the rare well-meaning ones, seem to radiate an evil aura or influence that can corrupt their surroundings, cursing those that might be nearby.
Ganondorf is the most well-known example of a mortal being who was corrupted through power and was later transformed into a great demonic creature that tried to take over Hyrule. In Ocarina of Time it was through the Triforce of Power that he gained the necessary power to transform himself into a large pig beast with horns and claws to attack Link. It was from this point that Ganondorf became known as the Demon King Ganon. Later on in Four Swords Adventures he gained a cursed trident that transformed him into the Great Demon King Ganon. Both transformations are similar in that the mortal Gerudo Ganondorf was corrupted by a very powerful magical artifact or item and turned into the familiar blue pig monster. In A Link to the Past’s story, he then went on to rule a separate world called the Dark World where demons and monsters roamed the land. He shared many traits of the other Demon Kings from the series in that he was an immensely powerful magical being that was nearly immortal and could not easily be harmed by common weapons. He lost his humanity in his quest for power and was forever transformed into something altogether evil and terrible.
Malladus was a great Demon King of ancient times that was eventually defeated by the Spirits of Good and sealed in the Tower of Spirits in the land that would later be named New Hyrule. He was described as a powerful demon that fought a war with the Spirits of Good after rising to power and destroying the land. He takes the form of a corrupting spirit that possesses the body of Princess Zelda, and later, after possessing Chancellor Cole, a gigantic blue monster with red hair, horns, and claws. This form is very similar to that of Ganon’s beast forms in Ocarina of Time and Twilight Princess.
The more well-known example of a demon, Ghirahim, or Demon Lord Ghirahim, was for the majority of Skyward Sword the primary antagonist and one of the servants of the greater evil Demise. He is a very vain and overconfident individual who at first very closely resembles a hylian. Later on his true nature comes out as a vicious killer intent on reviving his master. He was very contemptible of humans throughout his appearances and constantly looked down upon the mortal Link. He was also capable of shifting his appearance to that of a sword for his master to use, with several progressively less human forms separating the two.
Demise was known as the source of all monsters and the demon responsible for destroying the previous civilization on the surface world. Long before the time of Link, Demise and his minions came from a crack in the earth and devastated the land in their mission to claim the Triforce. He was defeated by the goddess Hylia, but not permanently. He was sealed within the ground but with his vast power, he could eventually escape to terrorize the surface one again. He has shown that he also has the same capability of transforming his shape, since he is stated to have a different form depending on who is looking upon him at the time. Whenever he broke free of the seal he would take on the shape of a giant monster with a huge gaping mouth, later on evolving arms and a dark halo that granted flight. Even in this sealed form, deprived of much of his power and intelligence, he was still too much for Link to defeat with the Master Sword alone. He still required sealing into the ground by using the Sealing Spike each time.
There are numerous appearances of other creatures defined as demons like the aforementioned Batreaux and Chancellor Cole. The bokoblins who have been seen alongside the moblins and bulblins in past games are alluded to being lower level demons in Demise’s army, based on their allegiance to Ghirahim in Skyward Sword. The vire seen in many games are called malicious devils, and Bongo Bongo of Ocarina of Time is a spirit that was sealed away in a well, just as many other demons in the series have been sealed.
In theology, a demon is a mystical and malevolent being that is known for their evil tendencies and dark origins. Built upon these real-world concepts of malicious beings that plague mankind in lore, in games they often form a much greater threat to the hero and the people he must save than the standard foe. Sometimes there is the occasional good demon found in literature and myth, but still the term mostly refers to an evil being that serves as the enemy (and even the good demons in Zelda seem to have an evil influence they cannot control). Most often their appearance in media is directly drawn from their depictions in folklore around the world.
In western media, demons are drawn from Christian literature, where they are fallen angels; beings who were once angels in heaven who later sided with Satan and rebelled. After this rebellion they were cast into hell and became the demons they are known as today. They spend their time plaguing mankind on earth and tempting people to sin. They wish to cause trouble and misery wherever they go. Other times they may take the form of beasts themselves and attack people directly. But overall they are always pure evil and irredeemable.
The Japanese have a somewhat different point of view and belief system to draw upon when translating demons to media such as anime or games. While some demons are still irrevocably evil and appear as the primary antagonists, there is the occasion when they are not. Japanese mythology allows for a somewhat greater range of emotions and motivations in its demons. More well-known creatures such as the Akuma, Oni, or Tengu have so many different interpretations and stories involving them that they can more easily be given different roles and even be beneficial to the protagonist of the story. This is in stark contrast to the more rigidly defined western system that leaves them as evil and corruptive at all times.
Demons and devils commonly appear in modern fantasy literature and games. They most often take on the role of the absolute evil entities and can take on the appearance of a gigantic monster of unearthly proportions or the beautiful and charismatic tempter who coerces the hero into evil through trickery. Again, they come in all shapes and sizes, most often not having any uniformity between them. But almost all originate from an otherworldly realm set apart from the mortal world the heroes come from.
With the proliferation fantasy books and games, many stories have taken a page from mythologies around the world to explain the origins of the setting. The demons of fantasy settings are typically included in these origin legends. Often demonic entities and spirits are manifestations of the primordial chaos that was left over from the creation of the universe, taken shape as malicious creatures. This is shown in Dungeons & Dragons, which is the template many fantasy stories follow. And this again takes from mythology where many greater monsters are actually primordial beings that can be older than the world itself, such as in Hinduism and Buddhism. Most often they represent the absolute evil that give the heroes a greater purpose in a greater conflict against pure evil.
Demons are such a common element in not only fantasy, but in real-world mythology as well, that it is no surprise that they have made so many appearances in the Zelda series over the years. Their mysterious origins and deep connection to the mythology of the world of Hyrule secure them as one of the most mysterious and most interesting additions to the franchise. I can only hope that one day we learn a little bit more about their origins and their relationship to the gods. Maybe the next sequel might shed a little more light on them.