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The novitiate Assassin undergoes endoskeletal restructuring, increasing his stamina and aerobic performance and allowing him to fight for days without tiring. Muscular hypertrophy enables the Assassin to explode into a fury of activity that lasts for hours at a time. In order to cope with these changes, the Assassin is given a second heart. Not only does this give the Assassin extra survivability should one heart be damaged in combat, but also ensures that when he is really driving his body to the limits, both organs can kick in, maintaining the excessively high heart rate demanded by his frantic killing frenzy.
If the trainee’s body survives all of the implants and modifications, his cerebral cortex is also enhanced by genetically coded lobo-chips. These chips amplify the Assassin’s state of mind, turning a mere dislike for the enemy into raging hatred, or the wish to serve the Imperium into determination to complete his mission at any cost. Adrenal ducts are implanted and electro-elixirs injected, speeding up the brain’s higher functions and allowing the Assassin to make complex calculations and tactical decisions in seconds.

"Dataslate: Officio Assassinorum"