During "recruitment" an astartes undergoes a series of painful surgeries to augment new organs into the body.

These organs increase to capacity of the individual to cleanse the heretic, the mutant, and the alien, beyond the means of any normal human.

You have exceptional resilience and mental fortitude, you have advantage on CON and WIS saving throws and immune to poison and fear conditions.

You have implants to augment you vision and hearing, you have darkvision to 60 ft and advantage on perception checks.

Belisarian Furnace (The Revitaliser) - This is a dormant organ that connects to both Astartes hearts. In times of extreme stress, or should the warrior's body undergo violent, damaging trauma, it expels great bursts of self-synthesized chemicals, once per long rest if you drop to 0 hit points, you instead drop to 1.