The Iron Casket, is what they call the Dreadnought Mech.
It's where an Interstellar Warrior is roughly stuffed into an alloy life support pod from which they can never leave again. All the life sustenance for this Interstellar Warrior will henceforth rely entirely on this life support pod for the rest of their life.
At the same time, this life support pod is the cockpit of the Dreadnought Mech. Numerous tubes connect to the Interstellar Warrior's nervous system, allowing them to control the massive Dreadnought Mech as if it were their own limbs. The mech's visual system becomes their eyes, and the auditory system becomes their ears.
It might sound not too bad.
