Demons come in a wide range of entities, varying in power from minor physical beings with the barest magical power to some of the most powerful entities entreated by sorcerers for power.
The first kind of demon are known as Elder Gods, and include the Elderspawn as well. Over a billion years ago, the primeval Demiurge (quite possibly related to the Phoenix Force) seeded the Earth, causing the first Earth-native beings of power to spontaneously spawn from the soil. These beings included the four primary Elder Gods – Gaea, Chthon, Set, and Oshtur – as well as many others.
Set and Chthon discovered means of gaining power by consuming other life forms (a practice known as necromancy or black magic), and with other Elder Gods who listened to their teachings proceeded to consume most of the other Elder Gods. This practice caused most of the Elder Gods to degenerate into demons. Gaea mated with the Demiurge and gave birth to the Demogorge; the Demogorge proceeded to destroy or consume the demonic Elder Gods; only Chthon and Set survived this purge by fleeing to pocket dimensions. Gaea and Oshtur, because they had refused to engage in necromantic arts, likewise were permitted by the Demogorge to remain.
Gaea soon merged her essence with that of Earth. She then mated with the Demogorge, giving birth to a number of other gods. In this manner, Gaea eventually became the progenitor of the pantheons of gods that have been worshipped by mortals, including but not limited to the Aesir and Vanir of Scandinavia, the Olympian gods of the Aegean, the Annunaki of ancient Mesopotamia, and the Heliopolitan gods of ancient Egypt. After a time, Oshtur decided to explore the universe and left Earth.
Other Elder Gods besides those four exist. Hoggoth of the Vishanti is believed to be an Elder God of the Kree. Ymir and Surtur of the Asgardians are also Elder Gods, having formed independently of the Elder Gods of Earth in Nifflheim and Muspelheim respectively.
Other known Elderspawn are: Agamotto, Earth's first Sorcerer Supreme and son of Oshtur; Tiamat, daughter of Set and co-progenitor of the Annunaki; and the N'Garai, the Other, and the Dwarf, spawn and agents of Chthon.
Each Elder God is said to have a role or personality that affects the grand scheme. Gaea creates and nurtures; Set consumes. Oshtur teaches white magic, marveling at and delighting in the unpredictability of mortals; Chthon teaches black magic, marveling at and upset by the unpredictability of mortals. Surtur cleanses with fire; Ymir dies so that life may thrive, only to constantly be recreated. And Hoggoth protects.
The second kind of demon, the Hell Lords and their kin, also originated on Earth, but come from later generations. When the Demogorge was done destroying or consuming the Elder Gods, he attempted to replicate the Demiurge's feat of seeding the planet with additional life. However, because it had consumed malevolent and parasitic beings in the course of its duties, the energy is spread about was "tainted". As a result, most of this second group arose as demons, although a few rose as gods. Most of these eventually left Earth to various other dimensions (typically Hell dimensions).
Chief among these demons are the demons presently known as Mephisto, Lucifer, Satannish, and Lilith; lesser demons are either born/created from these or are gods who have degenerated into demons. Many of these Hell Lords, as they are referred to in a group, have claimed to be the Judaeo-Christian "Devil", although whether any of them are in fact the Devil rather than taking advantage of human belief is unknown. Other Hell Lords have posed as gods of death such as Hades and Hela, tarnishing the reputations of these gods among mortals. In addition, various magical principalities such as Ikonn, Cyttorak, and Watoomb are believed to come from the second spontaneous generation who never degenerated into demons, although this is as yet unconfirmed.
A third type of demon has no immediately apparent connection to Earth, originating in mystic dimensions far from Earth. These demons include but are not limited to Nightmare, Dormammu, Aggamon, Tiboro, and the Mindless Ones. Like the Hell Lords, these demons often have lesser demons serving them, although most have mortal servants as well.
Various entities that claim to be the Judaeo-Christian "God" and angels that serve them also exist, although like the Devil, it is unknown whether any of these are in fact the God or just impostors taking advantage of or encouraging human belief. If the Judaeo-Christian God does in fact exist, he (or she) is possibly a either a surviving Elder God or is one of the later generations of gods. Some demons are known to be angels who have degenerated into a demonic status; more rarely, some angels may be demons who purged themselves of their demonic taint.
First Post-Reboot Appearance: DOCTOR STRANGE: SORCERER SUPREME #1.
There are dozens if not hundreds of species of demons found in various Hell dimensions, from the classic imps and succubi to the various demons described in literature, role-playing games, and computer games. It is recommended for GMs wishing to have demons as a regular occurrence in their games to use GURPS Dungeon Fantasy Monsters 5: Demons to help develop them. Vulnerability to Iron (including the organic steel of various characters with Body of Metal!) is a common Disadvantage among Marvel demons, though this is not an absolute.
Demons may be magic users, primarily using ecocentric and necromantic magic, but again, this varies by the type of demon and occasionally by individual demon (see Appendix: Magic). For a canon example, the demons of Belasco's Limbo – with the exception of N'astirh – including Belasco's and Illyana's chief enforcer S'ym, are primarily physical brutes, while N'astirh was a rare sorcerer demon. In essence, the Hell Lords and chief demons of extradimensional origin are magicians, while their underlings tend not to be; a magic-using "lesser" demon is probably going to be a noteworthy NPC, rather than one of the nameless hordes.