Program 257 Abyssal

Using data gleamed from the Spartan-IV program, Abyssal was devised as a means of creating sleeper agents to infiltrate the UNSC to spread Demiurge influence and control. Most their augmentations were directly taken from stolen from the UNSC, or reverse engineered from captured Spartans, dead or alive. Such augmentations were further teamed with an explosive kill-switch, and biochemical control systems to enhance aggression and alter brain chemistry to enhance performance, reward the subject, or punish them. Teamed with strenuous indoctrination, the sleeper agent is utterly loyal to the Demiurge, a deadly combatant equal in abilities to a Spartan-IV, and could be remotely controlled. 
Ryuko Kawada received augmentations of this nature, though many of the control systems, and kill-switch, were removed. Other such recipients were not so lucky, and many died during the UNSC's bloody, and secretive purge.