Die Ewigkeit (エイヴィヒカイト, Eivihikaito) is one of the many major elements in Dies irae. It's a sorcery created by Karl Ernst Krafft with the intention to spiritually merge an Ahnenerbe with their wielder, henceforth granting them superhuman capabilities. This is a form of sorcery that's possessed by most characters in Dies irae.
Most form of sorceries use lifeforce as their fuel, and it was rather common that most sorcerers back in the ancient era would use the blood or even the soul of a living thing (particularly human soul), to power up their craft. The phenomenon of an Ahnenerbe choosing their wielder and seeking souls as sustenance was not uncommon ever since the ancient times. These cases however, always end up with said wielders being destroyed by their attraction to their own Relic. In other words, there are next to no benefits for someone to wields a Relic.
Ewigkeit seeks to reform that. As its creation allows a wielder to form a symbiotic relationship with their Relic. By taking a vast amount of souls, the wielder also grows stronger in return. The wielder also gains lifeforce equivalent to the amount of lives they devoured, which vastly augment their physical capabilities while making it harder for them to die. Turning them into what's called as an Apostle.
Though essentially making them immortal and superhuman, that doesn't mean a group of human equavalent to the number of souls devoured can kill an Apostle. Possessing a hundred souls mean having a hundred times more lifeforce than a normal person. As such, the strength of an Apostle is fundamentally different than that of a human; as such, not even the strongest of military weaponries can lay a scratch on them.
Power and abilities
Aside from the aforementioned capabilities to devour soul and the physical augmentation that came from it, Ewigkeit also imbues the user with many other powers as well. Ewigkeit also allows an Apostle to sense and detect souls over a very large distances even down to their characteristics, attacks from an Apostle will also deal both spiritual and physical damage; forcing the reciever to defend themselves in both fronts, it's also possible for them to regenerate had they been injured or even reforming their body instantaneosly had it been destroyed until there's no trace left as long as their soul is intact.
Due to the thoughts and emotion that made up a Holy Relic, attacks from an Apostle will also exhibit some esoteric properties. The superimposed history that a Relic possess acts as a deadly poison that would immediately kill an ordinary person and inhibits regeneration. Furthermore, Since what it damages is the very essence of a human; which is their soul. Regardless of absence of pain and body status, just by slightest touch it can kill anyone. Thus, even incorporeal spirits can't survive an attack from an Apostle.
However, this power is not without flaws. As mentioned above, the most basic principle of Die Ewigkeit is absorbing the soul of other. Absorbing the soul of other beings mean absorbing all of the informations which are stored inside their soul, which includes their memories, emotions, and personal experiences. All of these informations would overload both the mind and body had an Apostle is of the weak of heart and body. While it's also possible for an Apostle to use this vast amount of informations for their own means, a tremendously high computing capability is required to handle such a large body of informations simultaneously. As such, this ability is almost useless in the course of a battle, as one would risk to lose even their own identity in the whirlpool of informations (unless it's someone with a huge ego, such as Reinhard, or someone's absolutely crazy, such as Schreiber).
Furthermore, an Apostle has a limit on how much souls they can contain within themselves at any given time. Taking more soul past their maximum capacity will start overloading their body akin to a wine spilling from an already full glass. Since an Apostle only possess a limited amount of souls, they must use them efficiently so that their fuel won't run out. In the circumstances wherein an Apostle ran out of souls and they don't have the mean to refill their own stock, they became no more superhuman than an ordinary people, such as the case with Wilhelm Ehrenburg near the end of Dies irae: Interview with Kaziklu Bey.
Levels of Die Ewigkeit
Assiah
The first level of the Ewigkeit formula, the characteristic that indicates the Soul Relic has manifested from the user’s soul. At this rank it is physical and somewhat unstable while at the same time giving the user a minor set of abilities. It also grants a more resilient body but can still be killed by hitting vital areas like the heart and brain. What a Holy Relic does varies from individual to individual.
Yetzirah
The second level of the Ewigkeit formula, when the Holy Relic is finally starting to show their particular traits. The Holy Relic is now stable and can be weaponized. His or her body is now strengthened to superhuman levels, as well as no longer able to killed through their vital areas and their bodies seizing to age, although the soul will continue to do so.
Briah
The third level of the Ewigkeit formula, which allows the user's absolute deepest desire to be made reality. There are two different types of Briah: Gudou (Which is when one's desire compresses inwards i.e. themselves) and Hadou (When one's desire compresses outwards i.e. others). Said desire gives the user an ability relating to it in at least some capacity. When the user has a Hadou desire, their ability will alter their surroundings, while a Gudou desire will alter themselves, though mixtures of both exist, such is the case with Wilhelm.
Aztiluth
The final and ultimate level of the Die Ewigkeit formula, when the user becomes a full-blown God and starts to paint over reality with their own Law, essentially making the universe/multiverse a canvas. The ability given to one by their desire is now amplified to far greater levels (ex. Schreiber being able to bypass the very concept of speed to go faster than his enemy), and users are capable of manipulating reality on a conceptual level, in addition to literally being walking universes, a mere drop of their blood weighing more than a neutron star. The difference of Hadou and Gudou still exists, now in the form of Hadou and Gudou Gods. However, not everyone is capable of attaining Atziluth, as it is a right determined at one's birth.