Arma Mortis (a.k.a. Armis Mortem) is a tome written by unknown angel in Reaper ranks, describing the research of Malthael on human nature, their souls, and the ways to weaponize them.
It reads as follows:
The malleability of mortal soulstuff is remarkable. I wonder to what other ends I might put it.
Within the breast of every human on Sanctuary lies a mote of pure darkness, proof of the breed’s demonic parentage. I suspect there may be a way to draw out that evil and contain it. Of course, the process is fatal, but I propose death is infinitely preferable to the life of doubt, rage, and confusion this darkness engenders.
I believe that just being in close proximity to the dark power within human souls induces an entropic effect in other materials. To be certain, though, I must test my theory by exposing material to the aura of the soulmetal I’ve forged and then inserting the exposed material into a living subject. A ranged weapon would make the ideal delivery system.
I discovered I might bind the darkness of the human soul into metal armaments as they are being forged. Evil intent literally roils off the metal, and I've never wielded anything so wickedly sharp.
The more subjects from whom I extract souls, the easier the process becomes.
It occurs to me that human souls might be alloyed with more than base metal. Their essence could likely be contained within energy matrices as well... or perhaps even within the light and harmony that makes up angelic forms. If those energies could be controlled, they would prove a source of immense power.
I realize now I’ve made a terrible mistake. The knowledge that the power of human souls might be extracted and used to empower the angelic Host would immediately escalate the Eternal Conflict. And if demons were to learn these methods, too... I daresay the war would end in our mutually assured destruction. All life would be extinguished. No one must ever know what I’ve discovered.
In-game
Arma Mortis is available as a Mystic plan given to players who purchased the Diablo III: Reaper of Souls Digital Deluxe Edition or Collector's Edition. It provides seven weapon and off-hand Transmogrification plans when used at the Mystic, all themed after Reapers and their artifacts:
- Quietus (Hand Crossbow)
- Mortal Coil (Flail)
- Deathreach (Fist Weapon)
- Sacrificer (Ceremonial Knife)
- Heart of Darkness (Source)
- Lifebane (Sword)
- Grim Reaper (2-Hand Mighty Weapon)
For centuries, this tome of weapon schematics was locked away, the items it depicted deemed by its author as too dangerous to ever see the light of day. During the confusion after the reaper invasion of Westmarch, however, the Armis Mortem somehow made its way into your hands.