...so this is what happened when some friends and I decided it was a good idea to mix Generator Rex with Rise of the Guardians. Over at the game where I play, the longer one is in the City of Change and the higher one goes in terms of Ascension, the more power one has, some of which has to do with the ability to alter reality, bending it to one's will. When Pitch Black arrived in the city, fresh from his defeat in his own world at the hands of the Guardians and their young allies, he was determined to establish himself. Everyone would know his name. Everyone would have to recognize his power. Newly Wakened, however, he lacked the power to create even one Nightmare.
So he learned to bend and he began to scheme. And, in doing so, learned that one of the city's Ashura, Rex Salazar, had memory issues. If Rex was wounded badly enough, his nanites would turn him into a rampaging mechanical monster, the side effect being total amnesia. So Pitch caused Rex a nightmare one evening, one that had him wake in such a panic that he was wounded in the process. His nanites went out of control and the reboot process began. In order to try and save both Rex's memories and the Southern District of Nautilus, Six fulfilled his promise to White Knight and killed Rex before the transformation could complete. But in the City of Change, will is everything...and even death can't destroy the willful essence of the living.
When Rex was resurrected via one of the gates five days later, his memory had been wiped (a temporary condition in that case, Six had killed him before the damage could become permanent). Pitch Black was waiting for him and when the EVO stepped through into Nautilus, the King of Nightmares laid out a spread of half truths that led Rex to believe his own family had betrayed and killed him because he was a monster in (mostly) human guise. Pitch then convinced Rex to join him, stealing him away to "protect him" from the other city residents who might do him harm, should they learn that he was alive again. The manipulation continued over the next several days and by the time it became clear what Pitch had done, Rex had changed his last name to "Black" (taking Pitch as his adopted father) and sworn his allegiance to the Boogeyman. With Rex's Ascended status and bending power to back him, Pitch created an army of Nightmares with which to invade Nautilus and show the inhabitants the true meaning of fear.
Rex became quite the terror during that time, using bending to alter his features so they could fit his more monstrous mentality. He even learned to mimic some of Pitch's powers with high level bending.
In the end, the city's senior Ashura led a strike team to invade Pitch's headquarters. They battled Rex in one of the underground caverns and captured him, the event traumatizing enough that it caused the return of some of Rex's lost memories. Pitch escaped, but only after a duel with a former ally left him bleeding and defeated.