Demons in the Marvel Universe

Demons are fictional characters appearing in American comic books published by Marvel Comics. The Marvel Universe hosts a number of demons, many of whom are at times at conflict with Gods and Angels. All possess varying degrees of mystical power, feed upon souls for self-sustenance, and are inclined towards dark or “black” magic - evil.

During the era in which the first single-celled organism formed on Earth, Demiurge achieved consciousness after being formed and coalesced from the Earth's biosphere. As conditions on Earth began to support life, it became aware, and wanting to know itself, the Demiurge split itself into countless aspects, each of which wanted consciousness and to manifest into new entities. These entities later become known as the "Elder Gods".

Most of the Elder Gods later degenerated into demons including:

However, certain demons, such as Lucifer, Kazann, and Asmodeus, have other origins, such as originally being angels having fallen from Heaven after a massive failed insurrection against the Judeo-Christian God long before the dawn of humanity, or hailing from other mystical dimensions adjacent to Earth.

It is also not uncommon for various gods worshipped throughout Earth's history to degenerate into demonic beings, though the distinction between gods who have degenerated into demons and gods who are merely malevolent is often unclear. For example, during the Fear Itself story arc, Cul Borson, elder brother of Odin, is the Asgardian God of Fear and uses the terror experienced by mortals to restore his youth and vitality, and to gain cosmic levels of power, but he isn't classified as a demon or having degenerated from godhood into a demonic existence. However, gods who typically use the life force of other beings, either through personal offerings or as some type of sacrifice, to sustain their powers are usually classified as having degenerated into demons. Aside from the various Elder Gods of the distant past, certain members of various modern pantheons have degenerated into demons. For instance, a large number of the Annunaki are now demonic including Ba'al, Nergal, Ningal and Dagon.

...
Wikipedia