The everliving. The ones who started it all, beginning in the early fifth century. The first recorded Immortals were the result of viral wipe out and human adaptation. There are several different ways for one to become an Immortal. You can be born one, a descendant from the Corvinus or other lines, or it could be self sustained, through various adaptations particular to the person.
Immortality is due to a bacteria, one that by genetic flaw, wipes out human existence, it's also been called a plague. Similarly, AIDS is a virus that affects many and it changes and molds to each individual person, Immortality does this as well. The bacteria changed Alexander Corvinus' (along with various others who's villages were affected by the plague) state from mortal to Immortal, it was a catalyst for the change. Because bacteria as a colony lives potentially forever, this grants the infected Immortality. This is passed down through the generations for those who are born Immortal. The onset of Immortality only comes after puberty has been reached. The surge of hormones from puberty triggers the bacteria to catalyze a chain, boosting the immune system, causing faster healing and reconstruction (overcoming age, disease and trauma) thus making the person Immortal by genetics.
Immortality has also been achieved through other methods. Gifts from the Gods, herbal remedies and genetic alterations from outside sources.
The Corvinus strain is the rarest of all the Immortal strains, and is the only one with the ability to create a Hybrid. Meaning that ONLY individuals who are descended from the Corvinus line are capable of becoming a Hybrid. It can also be a dormant gene, meaning the carrier cannot be an Immortal until they're infected by either a Therian or a Vampyre (as was the case with Micheal Corvin).
Homo Sapiens Sapiens: Mortals.
Homo Sapiens Aegrotus Immortalis: The very basic of the Immortals. Human in every aspect except for their genetic make up. Preventing them from dying of old age and disease; they heal very rapidly from most forms of trauma.
Homo Sapiens Aegrotus Corvinus: Descendants of Alexander Corvinus.
Culture and Hierarchy
Alexander Corvinus is the most noteworthy Immortal in the Underworld. Within their culture, the Immortals are shown to be very much kept in tune with the finer aspects of the Civil War the Lycans and the Vampyres wrought with themselves, among any other occurrences. The Immortals act as the barrier between the natural and the supernatural. They have kept the lines distinctly clear and when outbreaks threaten humanity; Alexander deemed and taught that all of the immortal species were oddities and that the world was meant for the mortals.
The only Hierarchy that had been in place, was when Alexander Corvinus himself was still alive, currently there has been no active individual to take over his or his team's rank, per se. Otherwise, Immortals are scattered into individual groups.