From the notes1 of Doctor Xerxes Orbizon Haliod2, renowned immortality researcher:

There seems to be a sort of natural order to beings, a progression of power with a number of distinct groups. Moreover, a firm affirmation of the concept of Evolution, to the keen observer. Others have different schemes and names for these, but most omit certain details that leave them inaccurate. My research has led me to the system outlined below.

Mortal – Sometimes referred to as Mundanes. Your natural, run-of-the-mill, typical being. Having mass completely subject to the laws of physics, and driven by mortal life-energy, known as Anima. Surprisingly, the vast majority of these are humanoid, many (including myself) even being outwardly indistinguishable from Earth humans. The apparent reasons for this are beyond the scope of these Notes (see Xenobiology Notes, Volume 16, Page 3744).

Supernal – This category of souls have amassed some small amount of power, enough so that the usual mortal weapons inspire very little fear. Usually this is achieved by becoming aware of and/or accessing the Anima within them, and in some cases, co-opting the Anima of others. Of course, just because they don’t inspire much fear doesn’t mean they won’t do some damage. Also, within Earth entertainment, the idea of supernal beings sometimes includes individuals who have surpassed this tier, as with the character Superman, who, while expressing great strength or some other powerful trait, are still mortal, composed of Anima, and therefore unable to truly effect a creature composed of Aether (see below), and subject to the eventual breakdown of cohesion subject to all Anima as it passes from this plane to the next.

Magus – An anomaly which has become rarer and rarer in modern times, Magi are those few mortals that have somehow obtained a small amount of immortality; some limited access to a source of Aether. They are not generally affected by mortal weapons. The line between a Magus and an Aeviternal (below) is difficult to define, and often to discern, but it does exist. To put a blunt point on it, A Magus can wield Aether, in some limited way. An Aeviternal is one with Aether.

Beings in the groups detailed above are not, strictly speaking, “immortal”. That requires complete infusion with Aether, at the very least. Only those groups listed below this point actually qualify.  I include the aforementioned groups in this summary simply for the sake of completeness, should this section of my notes be treated as the sum total of my thoughts on the subject.  Again.

Aeviternal – These illustrious beings have found some means to infuse themselves with Aetherial energy, the stuff of Immortality, which has made them impervious to death through any mortal means. Aeviternals – or Aeons for the uninitiated – remain immortal because of this Aetherial infusion, a steady stream of immortal life-force which protects the body from harm, and keeps cells regenerating (not to mention granting massive power and certain exemptions from the laws of physics). An Aeviternal may be slain only if the Aether that drives his life-force is depleted to an extent that it no longer maintains the body, which makes duels between Aeons long and violent.

Archon – Having had little exposure to Archons, I cannot say much about them, beyond that they are far more powerful than any Aeviternal – to the point where they begin to lose those parts of themselves which make them individual. Being vast and overwhelmingly powerful beings of luminous energy, they risk sundering the very fabric of our space-time whenever they enter our realm. Thus, they have unanimously opted to work in our realm through their Aeviternal Avatars. Archons are the highest form of being which still retain a complete sense of self, albeit a diminished one.

Aspect – It’s unclear if reality begins to align itself to creatures of great power, or if these beings naturally align themselves to fulfill the needs of the universe, but if a being amasses enough power, it may eventually become an Aspect. Essentially, they are the gears upon which our physical universe turns. Aspects are too primal, too fundamental to retain any true sense of self, and instead become the very concept of which they are an Aspect. Indeed, I have found no evidence that any Aspect is actually composed of separate beings; rather, each appears to be a single entity, entirely devoted to being the concept it represents, and incorporating all of the individual beings that compose it so absolutely that they are no longer a discrete entity. In other words, the parts are the whole, and the whole has no parts.

Deity – This tier seems somewhat obvious, but as these beings (many believe there is only one force of creation, and many believe that creation is driven by several beings. Nevertheless, one or many, there is more than one concept at play among the Archons, and the Aspects, though they may have greater insight from their vantage, do not deign to communicate downward on the matter. Therefore, I will refer to the concept of Divinity in the plural, so as to be inclusive) have only rarely been recorded as making appearances (and in fact my research has shown that these appearances are themselves actually performed by functionaries of the Archon or Aeviternal tier), they are the least easily defined, much less understood. They are creation itself. The hands that turn the gears of all of existence. Perhaps Coren’s summation is best: A deity is a God. Duh.

1 Specifically, Notes On Immortality, Volume 24, Page 17. This text is approximately halfway through the abstract of the work.

2 This spelling is an approximation; Ex (as his friends like to call him) uses the writing system of his native language when attributing his work, and there exists no direct analogue on Earth.