Dark World is a side story in the Little Witch Academia TV Series centering on Diana Cavendish. It takes place between Episode 16 and Episode 17.
Diana Cavendish awakens from sleep but learns that she has somehow grown into an adult. She explores her surroundings, learning that shes in the Forest of Arcturus. Living alongside are a coven of Witches including Sucy Manbavaran and to her surprise, her daughter, Bernadette Cavendish II. Diana inquires what was happening. Sucy, unaware what was happening to Diana, explains that the Witches were at war with the rest of humanity. Before she could learn more, the camp is under attack by Seishin, an organized group of shinobi who vowed to hunt down all Witches. An intense battle occurred but the Witches were losing ground. Sucy sacrificed herself allowing the other Witches to escape. Sucy fell under the mercy of the Seishin leader, Kabuto who proceeds to execute her.
After finding a new home to stay, Diana questions how all of this has happened and starts to investigate. With help from Bernadette, Diana learns that the war between humanity and witches started when Luna Nova was under attack by Seishin. It is later learned that Lotte was killed during that first attack, Jazminka was hanged alongside her family, Constanze was lobotomized after forced to built machines that locate Witches and Amanda and her resistance faction was buried alive. However, Diana wonder what happens to Akko and discovers an article revealing that Japan has been turned into a wasteland and the perpetrator who was held responsible was Shiny Chariot. Witnessing this cause Diana to have a flash when she first saw the show as a child. Meanwhile, Kabuto receives a report from her men about an anomaly unlike anything ever found.
Diana is worried about what happened to Akko and traveled to Japan, despite that the country has forbid any Witches to go there. She found Akko. As the two have a friendly conversation, Diana began to felt delirious just after drinking the tea Akko provided. She started to slumber but before she did, she saw and heard Akko giving an eerily sinister intention.
Waking up, Diana finds herself chained with dozens of Seishin guarding her. Diana realizes that Akko was working with Seishin but she was corrected by Akko that she's isn't just working with them, she was their leader, Kabuto. Diana begs Akko why she is killing witches. Akko reveals that long ago, Akko couldn't attend the show Shiny Chariot was performing hence her parents took her home. However on that fateful night, Shiny Chariot couldn't handle her magic and it unleashed a powerful flare that engulfed the entire country. Akko was one of the survivors but the death of her parents caused her to become the ruthless person she is today. Soon, Diana finally regained her memories. Having became overwhelmed with the knowledge that Akko was the one who will obtain the Grand Triskellion, Diana took matters into her own hands, fearing what future Akko might bring. She used a spell that allowed her to travel back in time and sabotaged the event that inspired Akko to become a Witch. Diana realizes to her utmost horror that she was responsible why Akko wasn't able to get the Shiny Rod and that the excess flow of magic from the portal was the reason why Chariot lost control of her magic, leading to the present where witches were driven to extinction.
Akko tortures Diana to learn the secrets of the time portal after learning that the anomaly was related to it. Having got what she wanted, Akko leaves to enact her plans; to travel to the past and murder the Nine Olde Witches before they ever discovered magic which would prevent the death of her parents and Japan. Diana pleads to Akko to not do this and reveals to her what the old timeline was like where everyone was happy. This enrages Akko even more and spits at Diana that due to that the Cavendish was here in the first place due to her own selfishness, it proofs that the world wasn't as ideal she make out and that fact alone is why Akko fully believes that the world will prosper without magic.
Diana would be rescued by Bernadette and Diana informed her daughter what was happening. The situation dire, Diana assembles any remaining Witches to mount a suicidal assault on the Seishin fortress. Diana and Bernadette have a touching moment with Diana expressing how proud she was of her daughter. The Witches confront Seishin and the battle commences. With the majority of Seishin distracted, Diana and Bernadette enter the fortress and confront Akko who was nearly completing the time portal spell. After Akko injured Bernadette, Diana unleashed her full might and sent Akko and her men into the entrance which allowed Bernadette to seal it, preventing them from entering.
Diana completes the spell and explains to Bernadette that she's using it to return to the past and restore everything the way it was. Diana can't guarantee that Bernadette will exist as she is the result of Diana's actions but Bernadette assures to her mother that they will meet each other as she believes. A miracle occurred and Bernadette was given the Shiny Rod. Seishin breaks in but Diana has finished the spell with Bernadette protecting Diana using her wand and the Shiny Rod. However, Akko slip pass Bernadette and attacked Diana when the spell was activated. Diana and Akko battled inside the portal ultimately leading to Diana to decapitate Akko leaving her lifeless head and corpse to disappear while Diana mourn over the loss.
Returning to the time she was a student, Diana found she was in the library reading the book about the time portal spell. She closes it and returns to her dorm room, thus preventing the dark future from ever happening. The next day, Diana meets with Akko and after a brief conversation, Diana places her blessings to Akko leaving the latter and her friends to be bewildered at what has just happened. Not long after time was restored, villagers found an unidentified corpse with its head nearby. That night, the village was in flames and the villagers were cut down by the headless corpse who walked towards to reclaim her head and walks into the flames. Diana's greatest mistake lives!!
Diana's actions were a direct result from the brief fight she had with Akko in The New Age of Magic setting up Dark World.
Diana experienced how costly her mistakes were. It is presumed that this lead her to leave Luna Nova but ultimately returned with help from Akko.