In the history of the Marvel multiverse, there have been few unions potentially as powerful as the one that was shared between one version of Franklin Richards and the X-Men's Rachel Summers -- leading to the birth of Hyperstorm. Born in a reality where the Sentinels had effectively taken over the world, Hyperstorm became a threat to entire realities.
Although he was defeated, another version of the character had the chance to become a genuine hero and even received a proven mentor in the form of a heroic incarnation of Sabretooth from the Age of Apocalypse in 2002's Exiles #12, by Judd Winick, Mike McKone, and John Holdredge. However, David Richards ended up becoming a uniquely dangerous threat, forcing one of the only good versions of Victor Creed in the multiverse to suffer one of his greatest losses.
The Sabretooth of Exiles was originally from the Age of Apocalypse. In that reality, Sabretooth had served at the behest of Apocalypse. But upon realizing how ruthless and monstrous those forces would quickly become, Sabretooth defected to the X-Men, the chief mutant resistance against the ancient villain. There, he became a surprisingly dependable and heroic fighter -- as well as a father figure to Blink, shaping her into a young woman capable of surviving almost anything. Both of them survived the apparent destruction of the Age of Apocalypse reality thanks to the interference of the Time-Broker -- in reality the Time-Breakers -- who sought their aid in repairing broken realities across the multiverse.
Initially placed on opposing teams -- Blink with the Exiles, and Sabretooth with the more morally dubious Weapon X -- the two teams encountered each other in Exiles #12. Coming together on Earth-2600, they were introduced to David Richards. A young boy living in a world dominated by Sentinels, the robotic killers had wiped out many heroes who called that reality their home -- including his parents, Franklin Richards and Rachel Summers. Thanks to his Omega-Level heritage, David possessed a number of powerful psionic powers -- including flight, power duplication, energy manipulation, healing, and reality-warping. He could even draw on the powers of the Phoenix Force, augmenting himself with a powerful energy shield.
Informed that David possessed the potential to become a nigh-unstoppable despot on his world spurned on by his resentment for the world and his sheer power, the two teams came to blows over what to do with him. Sabretooth was seemingly able to avert this future though by volunteering to remain behind on Earth-2600 and mentor David, helping him liberate his world from Sentinels but keep him from becoming a villainous figure. This thread was revisited in Exiles #59 by Tony Bedard and Mizuki Sakakibara. Over 20 years, David mastered his powers and became a pupil to Sabretooth. But the endless war took a toll on David, leaving him ruthless and cold. When Blink returned to visit the reality, David feared she was a spy and used his telepathy with little concern for her emotions or response.
Even after defeating the Sentinels, David used his abilities to control Blink and turn her into a weapon, using her and her powers to slaughter as many humans as they could find and coming close to breaking Blink's spirit completely. Sabretooth was finally forced to commit the murder he'd resisted two decades earlier, sneaking up on David and briefly depowering him with a power inhibitor long enough to rip the heart out of his chest. It's a surprisingly bitter moment for Sabretooth, who admits that while it hurt to kill his ward, it didn't compare to the suffering he'd brought to his adopted daughter. After learning of what happened to Blink, he couldn't stand the idea of the man he'd raised as a son, eventually fleeing this broken reality and officially joining the Exiles.