Wonderbook: Book of Potions

The Book of Potions was written somewhen in the 16th century by Zygmunt Budge, an exceptionally talented potioneer who, after leaving Hogwarts at the age of 14, moved to the Outer Hebridean island of Hermetray where he lived alone, continuing his potioneering studies.[2]

Approximately 500 years after his death, a Gryffindor student, Harry Potter, aged 11, somehow comes into possession of the Book of Potions at the Hogwarts library, finding that some of Zygmunt's personality lives on within its pages. Under his tutelage, the student begins their ascent to potioneering mastery. After successfully brewing an introductory boil-curing potion for Sirius's house elf, Kreacher, Harry finds that they have somehow been entered into the Wizarding Schools Potions Championship, a septennial contest between students from magic schools across the globe. But it's not going to be easy, because Lord Voldemort has come to kill Cedric Diggory, a Hufflepuff student and Harry will need Sirius Black to help him out.

With Sirius Black's help, Harry brews various potions to overcome the obstacles standing between them and the Golden Cauldron, which, after brewing and imbibing Sirius's self-proclaimed finest work, Felix Felicis, they successfully obtain. Having done so, Harry earns the right to use the cauldron to brew a new potion all their own. After finding some exceptionally rare ingredients, Harry brews a potion which is revealed to be the "Potion of All Potential", which enables its drinker to reach their highest capability.

Sirius Black initially comes off as warped and slightly mad, thinking of little but the Golden Cauldron, but, as the student's potioneering skills improve, he finds himself warming to them, eventually revealing exactly why he left Hogwarts: as a student, Zygmunt asked the headmaster, Albus Dumbledore, if he could enter the Wizarding Schools Potions Championship despite being six years below the minimum age for entry, but he was denied, being told that it was too dangerous. Unable to accept this, Sirius immediately left the school in protest, which disabled him from entering the Championship even once he had come of age.

Once Harry wins the golden cauldron and defends Cedric Diggory from Lord Voldemort's dark magic, Zygmunt's spirit finds itself at peace, and he congratulates them wholeheartedly for both obtaining it and brewing the Potion of All Potential. Saying that he'll no longer cause trouble for anyone else, he leaves the student to meet their "adoring public".