2. Dr. Light: From Pathetic B-List Teen Titans Rogue to Maniacal Serial Rapist

Originally: Dr. Arthur Light was a physicist turned supervillain when he gained the use of a special suit that allowed him to control refracted light. Over the years, he's feuded with everyone from the Justice League to the Teen Titans, almost always resulting in a pathetic defeat. Dr. Light was basically the poster boy for over the top, unsuccessful B-list villains with a gimmick that resulted in capture. The man was defeated by Little Boy Blue. Seriously.

The Retcon: During what I like to consider the birth of the contemporary DCU, Identity Crisis, writer Brad Meltzer retcons Dr. Light's history, making him the focal point of the events of the mini-series, which ultimately led into Infinite Crisis and beyond. In the story, Meltzer reveals that Dr. Light sneaked aboard the JLA watchtower at some point in the group's history, and found an unsuspecting Sue Dibny, the wife of the Elongated Man. Light savagely raped her, but was ultimately discovered by the JLA, and a decision was made. The group voted (not without argument) to use Zatanna's powers to mind wipe Dr. Light, altering his personality and predatory sexual behavior, hoping to eliminate the threat to their loved ones. However, Zatanna at the time was still learning to control her powers, and accidentally lobotomized Light, leaving him as the bumbling idiot readers had come to know for decades.

The Impact: This massive retcon of a previously insignificant character changed the landscape of the DC Universe, and for the better. First and foremost, this opened the eyes of readers to a more sinister and fallible DCU, where their favorite times of the past all turned to dust. More importantly, this single event led to major dissension within the JLA, as when Batman found out what was done, the team opted to mind wipe him as well, betraying their own. Of course, Bruce remembers it all at some point down the line, and pushes him to become the distrustful man we all know and love, relying more on himself than anyone else. He then builds the Brother Eye satellite to spy on, well, everyone. Brother Eye would go on to play an important role in Infinite Crisis, and the mind wiping scandal would keep popping up throughout the next few years. Dr. Light would also go on to recover his memories, like Batman, and seek revenge on those involved. He also would admit that Sue was not his first victim, thus cementing his place as one of DC's most heinous villains. The retcon of Dr. Light's character is thought provoking in every way: it raises the question of the moral lines that these heroes abide by, as well as the self justification of one being a hero and taking responsibility for others. What constitutes one person to be a hero? Certainly it's not simply a gift of super powers, because as we see in Identity Crisis, even those with the best intentions can cause havoc.