Alternate reality

Alternate realities are versions of the prime universe where different events have occurred.

Harry Potter And The Cursed Child
Alternate realities play a vital role in Harry Potter And The Cursed Child, when Albus Potter and Scorpius Malfoy use a time-turner to save Cedric Diggory. However, in the process, they meet Hermione as a girl, passing off as Durmstrang students. This causes Hermione to get suspicious and she goes to the Yule Ball with Ron rather than Krum, thus the jealousy that started Ron and Hermione's relationship never existed. This meant that they never existed, making Rose and Hugo not exist in this timeline,a s Rom married Padma Patril instead. Their son Panju made albus want to be a Gryffindor. Later, when they try to fix it, another alternate reality is caused where Cedric killed Neville because he was mad for losing the Triwizard Tournament. This caused Delores Umbridge to remain Headmistress of Hogwarts. Since Umbridge remained, she helped Voldemort kill Harry, who she called a Dumbledore terrorist, as Nevile wasn't there to save the world. Voldemort took over. This also erased Albus from existence.