Apotheosis is the elevation of an individual to the level of the gods, a process that causes them to transcend their body and become an immortal magical existence that has surpassed death. No longer will the agent be bound by their body or mortality, with no need for any container. An agent who has achieved apotheosis can easily reform their body after its destruction, and can thus only be killed by critical damage to the soul. While many agents can leave their bodies as a soul to possess others, their soul is not immortal and still needs a container; without one, it will eventually fall apart.
On Earth, apotheosis was pursued by many branches of magic, such as Alchemy and Tao Chiao, but the road to perfection is a long, arduous, and complex one. To reach apotheosis, one must have a deep understanding of not just their body and how to recreate it, but on their soul and the nature of magic as a whole. Due to the abstract and complicated structure of the soul, this is incredibly difficult, and even when Earth's humans devised how to create an artificial soul apotheosis remained a distant dream.
It is unknown if anyone actually achieved apotheosis on Earth in the years leading up to its destruction at SOLSTICE's hands; in the years that followed the beginning of SOLSTICE's rampage, her ex-wife, EQUINOX, managed to achieve it for herself after years of research and by mixing practices and theories from many different branches of magic. Others followed, but even when the road to apotheosis is laid out it is difficult to follow; every person's journey to apotheosis is ultimately their own. Simply possessing knowledge is not enough, certainly - one must have enough of some indescribable, intangible “experience”, which may actually be more vital than research. Regardless, apotheosis is generally thought to be necessary to fully access the power of an inner world, as that is the point at which an individual's inner world is strong enough to stand up against the cruel weight of Reality.
There are beings that naturally exist in a state similar to that of apotheosis, in which they are not dependent on any body or container at all, such as loose spirits and angels. These beings cannot be truly said to have achieved apotheosis, as such a thing was never a concern for them anyways. Apotheosis is only relevant to mortals.
Many agents believe that apotheosis on its own is not enough, that an individual can perfect themselves further and transcend even the soul, becoming a raw conceptual being. This is often called simply second apotheosis, although other terms exist. Unlike apotheosis, which, while not exactly widespread, is something many individuals reach across the universe, second apotheosis remains something much more theoretical and mysterious. There are those who have transcended their souls, but the methods vary, as do the end results, while apotheosis is much more straightforward once one gets to that point.
One theory that holds second apotheosis in particularly high regard is the theory that souls and other beings were not originally connected at all. Proponents of this theory believe that, as mitochondria evolved from stray bacteria to become part of eukaryotic beings, souls evolved to become integral to the existence of most beings in not just our universe, but all universes. This connection would give the physical a greater connection to Fantasy, while it would give a form and container to the spiritual, allowing them to more easily influence the physical world.
Many who subscribe to this theory thereby believe that bodies die when their souls do because of a placebo (or, more accurately, nocebo) effect; because the individual believes on a deep subconscious level that they would die if this most abstract part of their self was destroyed. This theory says that, eventually, an individual will be able to transcend this aspect of their self and realize that their soul is not necessary. After all, the brain contains all the information that makes up an individual's identity, and a person can be completely changed by driving a pole through their skull.
Of course, theory is theory and practice is practice. While evidence can be pulled up, it has never been anything decisive, and it remains a matter of debate. Even if it were the case, it would not be easy for someone to convince themself that they don't need their soul. Plenty of tried, often even to prove the theory, and plenty have failed. Others have succeeded, but this means nothing; this is the power of a strong will, after all. Sheer force of personality can accomplish anything, as unreasonable as it may be.
Regardless, plenty of beings across the universe have managed to evolve past their souls in some form, becoming soulless beings. Many, but not all definitions of second apotheosis include such beings; the strictest definition is that second apotheosis applies only to those who have fully transcended body and soul to become raw conceptual beings, living ideas that exist only in an abstract form. Individuals who can be said to achieved second apotheosis include Mastema and Raphael, as well as Observer, though Observer certainly has not completed her evolution and achieved that theoretical true abstract form.
As with apotheosis, beings who came into existence without a soul cannot be said to have achieved second apotheosis. It is another matter that is simply not relevant to them.