Alternate realities are versions of the original universe in which different events have occurred, primarily due to disturbances created during time travelling.
Eloise Mintumble was a witch who time travelled from 1899 to 1402 for five days. Over her time there, she managed to change the life paths of several others which made it so twenty five of their descendants were never born, thereby creating an alternate reality.
Albus Potter and Scorpius Malfoy
Albus Potter and Scorpius Malfoy were two young wizards who travelled from 2020 to 1994, to try and stop Cedric Diggory from being killed by Lord Voldemort. When they created an alternate reality which hadn't turned out how they were hoping, it put them both on a path to try and correct what they have done. Ultimately, they created three more timelines before finally going back to a future that is identical to the original timeline.
Around the summer of 2020, Harry Potter found a new time-turner. When Amos Diggory found out and eventually went round to Harry's home to beg him to use it to travel in time and save Cedric Diggory, Harry told him he could not meddle with time and Amos left angry. Albus Potter heard the conversation and following an argument with his father on 31 August 2020, he decided to help Amos and travel in time to save Cedric. The next day, he told his friend Scorpius Malfoy of the plan and they found a way to jump from the Hogwarts Express on their way to Hogwarts.
Alternate timeline 1
Albus Potter and Scorpius Malfoy travelled to the First Task where they pretended to be Durmstrang students. They accidentally met Hermione as a young girl who followed them, and when she noticed them intervening with Cedric's task, she thought Viktor Krum had set them up. She went to the Yule Ball with Ron instead, but without the catalyst of Ron's jealousy over Krum helping him acknowledge his own deeper feelings for Hermione, Ron and Hermione never fell in love and Ron danced with Padma Patil. They fell in love, married and had a son, Panju Weasley, instead. Therefore, Rose and Hugo Granger-Weasley never existed.
Hermione grew up to be a mean Defence Against the Dark Arts professor, while Ron grew up to be a solemn man. Albus persuaded the Sorting Hat to place him in Gryffindor, and so it did, while Scorpius remained in Slytherin. Cedric also still died in the timeline. They avoid each other for a week before they decide they have to fix the timeline. Albus believed that by humiliating Cedric at the Second Task, and also by affirming Ron's feelings for Hermione, they would change the future to the way they had hoped. They then used the time-turner and travelled to 24 February 1995.
Alternate timeline 2
Albus and Scorpius intervened with the Second Task and changed the future to another alternate timeline. They managed to save Cedric by over-inflating the Bubble-Head Charm so that he was floating above the lake in his swimsuit, accompanied by a large fireworks display affirming 'Ron Loves Hermione'. However, this led to Cedric joining the Death Eaters because he felt so humiliated by the tournament. During the Battle of Hogwarts, Cedric killed Neville Longbottom, preventing him from killing Nagini, which, in turn, ensured Voldemort was not killed and gave him the opportunity to kill Harry and win the war, taking over the wizarding world.
Due to Harry's death, Albus was erased from the timeline. Scorpius returned to the future alone and found Hogwarts now under the control of Dolores Umbridge. This Dark Hogwarts was used to train future Death Eaters by teaching them the Dark arts by torturing and murdering Muggles and Muggle Borns in twisted experiments of dark magic, Dementors patrolled the grounds and The ministry of Magic was under the control of an individual known as the Augurey (Delphini) and Draco Malfoy was Head of Magical Law Enforcement, whose duty was to bribe the Muggle government into silence over the various atrocities being committed by death eaters such as blowing up bridges to see how many muggles they could kill and perhaps the most horrific outcome of this reality was the Muggle death camps burning alive anyone who opposed Voldemort. Scorpius would later go on to describe this reality as "HELL." Scorpius eventually joined forces with a small resistance and with the help of Severus Snape, Ron and Hermione, they tried to retrieve the original timeline. They time travelled to the First Task and used a Shield Charm to stop the other Scorpius intervening.
Alternate timeline 3
When Scorpius, Ron and Hermione returned to the future following their trip to the First Task, they found the timeline still very identical to the third timeline. They were attacked by Dementors, so Ron and Hermione sacrificed themselves to give Snape and Scorpius time to flee. Scorpius and Snape made their way down to the lake but when they arrived Umbridge was waiting for them but Snape managed to subdue her and allowed Scorpius to escape at the cost of his own life. He then time travelled to the Second Task, where his intervention retrieved the original timeline.
Albus and Scorpius decided to take the time turner and destroy it, and invited Delphi to the Owlery to help. Delphi stole the time turner and made Albus and Scorpius travel back in time with her to the third task. She wanted to try and save Cedric Diggory in the hope it would return Voldemort to life in the same way it happened in the second alternate timeline.
Albus and Scorpius managed to trick her and steal the time turner back with the help of Cedric when he stumbled across them and thought they were part of the tournament. When Cedric left, Delphi managed to get the time turner back and used it to travel in time again.
They travelled to 30 October, 1981. Delphi destroyed the time turner so they couldn't be sent back to the present, and left Albus and Scorpius, who eventually discovered they were by Godric's Hollow. They went to the home of James and Lily Potter and watched them going out for a walk with a one year old Harry. They thought Delphi was planning to save Voldemort by killing the Potters herself.
They decided they had to tell their parents in the future where they are, so they stole the blanket that belonged to Harry. They created a message on the blanket and treated it with a substance that would only become visible when it was exposed to a potion that Albus would accidentally spill on the blanket before the time travelling began, allowing Harry to read the message in the future. Draco told everyone that his parents had a time turner, which they used to travel in time to 31 October 1981. They found the children and Ginny realised that Delphi wasn't hoping to kill the Potters herself, but actually wanted to stop Voldemort from trying to kill the Potters in the first place.
They decided to try and fool Delphi into thinking Harry was Voldemort and transfigured him to look like Voldemort. It worked for a moment, but the appearance eventually faded away and revealed the truth. Delphi was angry and duelled Harry, who was helped by his wife, son and friends. They apprehended Delphi, who then saw Voldemort going to Godric's Hollow. She tried to get his attention, but was stopped by Hermione and Draco. Harry and Albus stayed and watched James and Lily die in the hope of finding closure, before they returned to the future. They ultimately returned to find a new present nearly identical to the original timeline.
Behind the scenes
- Injuries acquired during time travel age up in a different manner than the time-traveller. When Albus Potter was injured after travelling twenty years in the past, his injury aged twenty years (according to Madam Pomfrey) even though Albus himself remained the same age. Similarly, Eloise Mintumble may have been injured when she was retrieved by her colleagues, ageing her up hundreds of years.
- Knowledge of the un-born is retained by the people who made the changes in the timeline. The fact that at least 25 people vanished due to Mintumble's time travel may have been known only because Mintumble looked for them, or her retrievers retained such knowledge.
- Unique in Mintumble's case is that time itself was messed up - days were shorter or longer. But the disparity is too great to be comparable with Albus and Scorpius' time travel adventure.
- What happens to the alternate reality after the changes are "fixed" is unknown; it is possible the alternate universe still exists parallel to the main timeline.
- Some fans have theorised that the timeline in which Harry and Hermione rescue Sirius Black and Buckbeak is an alternate reality, but this is not true, since Prisoner of Azkaban focuses on a predestination paradox.