Magic is different depending on where one is, and the world in which they reside. There is a sort of universal magic that applies everywhere, its rules immutable and far stronger than any magic from which it descends. However, as elemental as that magic is, little can be done with it.

Universal Magic

All magic requires power, energy. Something cannot arise from nothing.
Knowing the name of something gives power over it.
All souls contain an essential vitality, energy, which can be used in the absence of any other power to fuel spells.
The transference of vitality from one thing to another requires, in at least some part, sacrifice on the part of the transferer.

A form of magic that operates entirely under the principles of universal magic is true names. Indeed, unlike all other forms of magic, the invocation of true names is just the practical application of the four rules.

Souls

The essential vitality that lives on even after death. Souls are what empower magic, the difference between a cantrip and a true spell being to what degree it uses the power of a soul. A magical act that would use less than the full breadth of a soul to cast is considered a cantrip; these actions can be performed essentially infinitely, without draining a soul of all its vitality. A spell, by contrast, will consume all the vitality of one or more souls in attempting to cast it.

When someone is alive or their soul is otherwise inhabiting a living body, their vitality will always restore to their capacity over the course of a full night's sleep. In the event that they are unable to enjoy a full night's sleep, their magic will not restore whatsoever. A character who is either projecting their soul outside of their body, or otherwise not inhabiting a living body, must find alternative means of restoring their soul

Transference

Anyone, no matter their proficiency with magic, if they have a soul they may choose to grant power to another. This power cannot be granted easily, however. For a consensual transference of power, it must be in the form of a sacrifice. These sacrifices can be as subtle as acts of worship, they need not necessarily be grand displays of power.

There is also, of course, transferences of power that are not consensual. This is often in the form of cannibalism. Eating another person requires no special magic, no technique, no practice. If someone is eating the body of another, then each morsel will imbue the cannibal with the essential power of one soul. If the cannibal eats the heart of their victim, then they have by this same token eaten their victim's soul.

Capacity

The capacity of one's soul is measured in souls. Because some souls are inherently more powerful than others. The worth of a single soul, for the purpose of magic, is thus:

A fireball, roughly 3x3x3.
Lifting and moving something weighing up to 100kg for one minute.
Shoving something up to 200kg back up to five meters.
Conjuring up to 1kg of material in any shape or configuration that the caster can imagine.

Antimagic

Characters with the soulless trait cannot use magic, unless the magic relies on a source other than their own soul to conjure or direct the energies involved. This is not quite the same as antimagic, though. The simplest and most powerful form of antimagic would be knowing the true name of a given system of magic. If one learns its true name, they have power over it; that power may then be used to deflect, or it could even unravel the entire system of magic altogether.

Countering

The more common form of antimagic is countering one spell with its inverse. Life cancels out death; fire cancels out water; and so on. The only caveat is that the spells must be of equal power, and actually come into contact with one another. A clever mage can bypass a counterspell by just maneuvering around it, while a powerful mage could simply overpower it.