Seikishin - also known as Sacred Mechalords in the Funimation translation - are ancient and powerful humanoid constructs that were built by an ancient race called the Senshi.
Seikishin were originally created for experimental purposes, and to show off technological ability. In time, they became used in sports and other forms of competition, including combat.
Eventually, the Seikishin reached the point where humans could no longer stand the stress of piloting, thus they created Artificial Humans.
Artificial Humans had great talent as pilots, but lacked understanding outside of combat, so often would misunderstand orders. It is believed that this was what eventually caused the Seikishin to go berserk, and begin to destroy the Senshi civilization, led by Gaia.[1]
Gaia, the only surviving Seikishin, was unearthed intact (although immobile) by the Shitoreiyu Empire, and was mainly used as a decoration before the arrival of Kenshi triggered a response from it.[2] It was revealed that the Senshi civilization was completely destroyed by Gaia.[3]