The original Fearless Mecha was prepared for dying Space Marines.
The Fearless Mecha is their rebirth, their body, and also their grave, their coffin. The living nervous system and electronic bio-fibers inside the Fearless Mecha connect the human nerves with the Mecha's magnetic muscles completely. Once a being completes the coupling with the Fearless Mecha, they can never leave the metal womb of the mecha; their fate is to fight until the last moment in this walking coffin.
Although the technology of electronic nerve bundles has advanced significantly later, once the coupling is completed, it is still very difficult to separate the human body from the Mecha.
Because this is originally the creation of Mechanical Ascension.
All warriors selected to be coupled inside the Fearless Mecha undergo a 'Spiritual Purification' ritual, meaning they no longer have physiological desires and food can only be supplied as nutrient fluid from the outside world.
