What are they and how to acquire them?
Armored Soul (or ArmS) basically are powerful spirits of unknown realm, sealed within the darkness that they refer as "the cage". Having no memories of their past before sealed, they spent aeon of time searching the purpose of their existence.
For some reason the cage itself could only be breached from outside through the dream state of people that having strong desire of power whether the reasons are noble or evil. From there depends on the ArmS personality, the test would be started which could take form of those people worst nightmare to something too good to be true illusions. Only if the person proved to have an unwavering will that the ArmS will swore their allegiance and gave their true name.
The pact is made when the Arms give their true name to the user. Starting there his or her body officially become the new cage for the acquired ArmS, which proven by the appearance of the summoning seal on their skin.