Upon Albus Dumbledore’s death at the hand of Draco Malfoy, Aberforth Dumbledore knows he must do clean up the mess his arrogant elder brother left behind. It just took him three years to accomplish his goal. In 1999 he used means that the Ministry would never approve of, if that can even be said anymore, Aberforth traveled back in time to 1980. His original intention was to kidnap Sybil Trelawney before she could make the infamous prediction, but he was too late. So he went after his second target: Peter Pettigrew. Kidnapping, imperiusing, and then oblivating the young man was certainly the least of his crimes, but it kept him from betraying his friends’ most sacred trust and from innocent blood from being spilled. Harry Potter would never become the Chosen One.
Unfortunately, fate always finds a way in the end. Since he could not go after the Potters, Tom Riddle went after the Longbottoms. Augusta Longbottom put up a valiant fight, but on November 5, 1981 Lord Voldemort found his downfall with her grandson, Neville Longbottom. While the Wizarding World hailed the toddler as the Boy Who Lived, the remaining members of the Order of the Phoenix began the arduous task of hunting down and destroying the remaining horcruxes.
Over the last year, the residents of the Wizarding United Kingdom were witness and victim to events that, at the time, seemed relatively harmless. Memories, feelings, and items were brought between the realities. What happened in canon was suddenly at the fore of people's minds. They knew things they shouldn't, remembered experiencing things that, because of Aberforth's actions, never happened. Dark Marks made appearances around the country, but no one at the ministry could pin down who or what was causing the events.
Barty Crouch, junior, spent the last year disguised as his father by ways of polyjuice potion. He was discovered on Halloween night when he tried to pull Death Eaters through the rifts in time. An employee of the time room in the Department of Mysteries caught him attempting to use the time turners to get back in time to stop Aberforth from destroying the Death Eaters. The employee apprehended BCJ, who was returned to prison on kidnapping and treason charges. He is scheduled to receive the kiss.
The employee returned to the time room curious to see exactly what, if any, trouble BCJ caused to the timelines and time turners kept in that particular chamber. A glance at the instruments didn't show anything was wrong, but as he stood there, time had a meltdown, and he ended up sliding in and out of time periods and timelines. Anyone he came into contact with during these trips didn't remember the meeting. He eventually returned home to learn that he was a bit of a hero, but that first trip made it possible for him to jump nearly at will while in the time room.
On a New Year's Eve jump, he caused a small rift between game timeline and canon. His return trip brought two visitors to the Redux world. The year is 2003. Voldemort is still dead. Neville is still the chosen one. Those we lost in the first war in canon are still alive. Two new friends have come to play, and their games aren't exactly family friendly. Their ideas for starting off a New Year don't involve diets or starting a new book club. Bellatrix Lestrange is not thrilled with this new timeline, granted she wasn't overly thrilled with where her's was headed either. But where there is a will (and a handy minion) there is a way. Setting up shop in her Knockturn Alley townhouse she had owned with her husband, she has a goal in mind.
If letting Fenrir run amok let her work to set things 'right' stay hidden, then who was she to complain? The Wizarding World is about to be turned on its ear, but no one knows who the new players are just yet. The longer she can stay in the shadows, the better, because Bellatrix would love nothing more than to have the wheels of her plans in motion to the point where they would be unstoppable.