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[DC/Marvel] What is Magic exactly in each universe, and who are its top users?

Is it divine (Gods and stuff), is it some sort of natural energy, is it something beyond comprehension?

Who are the big names in each universe who use magic, what type of magic do they use? Can anyone use magic in these universes or is it a natural-born gift?
gpt
Magic in the Marvel multiverse is a discrete type of energy. It's the energy that is generated by the will of sentient creatures, and is empowered to alter the fabric of reality.

Magic in and of itself is natural, as in it is a regular and normal part of the structure of the multiverse. There are lots of subtypes and specializations of magic, as well.

An important but powerful subtype of magic is divine magic, which is manifested in and passed down by divine entities, or gods. It's hallmark is that it is generated primarily by belief- the will of sentient creatures dedicated primarily to the edification and service of the deity. Much of Earth's history has been influenced by divine magic.

In the Marvel Universe, there's different levels of reality, and magic has sway over almost all of them. Very few humans can access much more than the very lowest levels and are unaware or unable to interact with any of the higher realities and possibilities without assistance.

For the most part, the true nature of magic is beyond comprehension though. This is why most systems of using magic are concerned with distilling it into spells or locking it into artifacts or objects, and finding other ways of codifying and limiting access to it so that normal minds can unlock its potential.

Of those that can, the greatest is the Sorcerer Supreme, primary guardian and wielder of magic in the 616 universe, Stephen Strange. 

Even he mostly uses spells and artifacts to access magic, so his power lies not so much in his pure ability, but in his breadth of knowledge of such spells and his proficiency in use of the artifacts he has access to, and the staggering number of artifacts he possesses.

There are some entities who are even more proficient in using pure magic, or are even composed of pure magic. 

For example, Dormammu is an entity who hails from an entire magical dimension, one of several creatures of that type called the Faltine- composed completely of magical power. He has access to all kinds and types of magic, but mostly simply uses pure magical energy to accomplish his goals.

The Vishanti and the Octessence are groups of extra-dimensional magical beings as well, and also use the entire spectrum of magic. They also are the primary sources of the power the Sorcerer Supreme uses to protect the 616 main Marvel universe.

The gods and deities that use divine magic have some of the most powerful realms in existence. Odin, for example, has power sufficient to stand with some of the most powerful forces in the Marvel multiverse. 

There are many entities who are powered by the emotional energy of sentient beings other than belief, as well. Those are called devils and demons, and they reside in realms that consist of that negative magical energy. There you will find Mephisto, Belasco, and such.

Hyper-dimensional creatures like 'the Old Ones' wield a sort of magic that is literally incomprehensible- it does not match with the ability of creatures in the 616 reality to perceive. Shuma-Gorath is a power in that fold.

You asked who can use magic in the Marvel universe. For humans and most humanoid aliens, the ability to use pure undistilled magic in the Marvel universe is usually an in-born trait. It can be manifested in humans through the Celestial gene that gives creatures superpowers, through the X-gene that powers mutants, or can be a hereditary gift passed down through the descendants of magical entities who bred with mundane creatures. 

Manifesting the spontaneous ability to use magic was much more common on Earth in earlier eons when magic was much more prolific and unhampered by the logical belief of science. In the modern age, it's almost unheard of for a human to be able to wield magic unprompted by outside forces.

The three most common ways that humans do get access to magical abilities is to have them given to them by a high-level benefactor, to somehow get access to a magical artifact, or to be educated in the craft of using magic by a mentor.