When it began the people didn't know what happened only a select few understood. The spirit realm was colliding with our plane. Unfortunately with the spirit realm being subsumed the hatred stockpiled untill it formed Aku the evil and rage personified who's only goal is to make the mortal realm pay any way and every way.
The power of Aku accumulated into armor ,animated armor in which he resided. In order to be able to fight back a old priest stole plates from his armor which was a lucky venture because he was smashed. He the priest figured he was boned and proceeded to drink himself stupid then go for it. For humanity's survival sake he did it. For the liquors sake he didn't remember how. Not to worry he made up plenty of stories to fill the whole in the historical records but never confirmed any of it.
From the peice of armor he stole he fashioned 8 armors for 8 different armored soldiers as there souls were not empowered to make there own armor this was the only defense of our plane against them. In the end they won, and in doing so shattered Aku and there own armors in the process.
This was the end untill the merger of planes ended and a higher calling came that other planes would call on them for war and profit. So started the the ARMORED SOUL era.