If you haven't read the first two installments of this trilogy, I advise you do so before reading this book. There will be a lot of things you won't understand that occur in the earlier books.
Previously In The Book of Shadows:
Book of Shadows Pt. I: Subject NE-001:
Everything happened as it was supposed to. With the exception of one tiny difference that changed the fate of reality. Instead of being held back by Remus Lupin after the supposed death of Sirius Black, Harry Potter took matters into his own hands and dove directly into the Veil of Death. The ancient structure acted as a portal between two worlds, dumping a defenseless and traumatized Harry into the hands of the Umbrella Corporation. Renamed 'Subject NE-001', a brainwashed Harry Potter would serve as a weapon for the secret organization for two years. But his time as an assassin for Umbrella would be put to an end with the viral zombie outbreak within Raccoon City. After an incident involving Harry's release from Umbrella, he stumbles into the former S.T.A.R.S member Jill Valentine, who is on her own mission to escape the city and survive the pursuit of the bioweapon known as Nemesis. Despite Harry's attempts to help Jill in her mission, the two would eventually be separated by an attack by Nemesis, which would end in the destruction of an entire city block.
After the separation, Harry ran into rookie cop Leon S. Kennedy and the young college student Claire Redfield, who were both attempting to survive outside of a local gas station. The three would work together within the Raccoon City Police Department in their attempts to escape the city. Due to complications and separate missions, Leon split off from the group in search of a way out through the city sewers, accompanied by the spy and assassin Ada Wong. The two would go through the sewers and would eventually find a way into the NEST Facility beneath the police department. While Leon faced the lies of Ada and the corruption of the scientist Annette Birkin, Harry and Claire worked their way into another underground facility. Once there, they discovered a hiding Sherry Birkin and her mutated father William. Harry, Claire, and Sherry managed to escape William's grasp, but not without Claire being knocked unconscious. To protect both Claire and Sherry, Harry created a portkey that would send them to safety, his new mission being to rendezvous with Leon in the NEST Facility. Harry was able to intervene during Leon's battle with the mutated William Birkin, easily killing the mutated man while destroying the dangerous G-Virus. After a brief betrayal and a quick evacuation, Harry, Leon, Claire, and Sherry were safe outside of Raccoon City. But with Jill Valentine still infected in the Hospital and with survivors still within the city, Harry and Claire decided to venture back to the Police Department.
Once there, the two worked together to rescue a number of civilians, as well as end the life of former police chief turned psychopath Brian Irons. However, after being attacked by a third mysterious Bioweapon, Claire and the survivors were forced to leave Harry behind in search of safety. The battle ended easily, with Harry emerging victorious while also reuniting with the now cured and walking Jill. The two worked together to acquire a vaccine for the T-Virus, and eventually both Jill and former Umbrella mercenary Carlos Oliveira were able to escape the city, leaving Harry behind once again in order to save the city from nuclear detonation as well as to spread the vaccine. He was successful in both, curing the city of the outbreak as well as preventing its destruction.
In the month that followed the outbreak, Harry and the others worked to rebuild Raccoon City to its former glory, with Harry leading the city as its Mayor. But after an alternate with the United States Army, Harry was forced to leave Raccoon City behind to protect his friends from harm. He took himself to an isolated region of Romania, where he reunited with his lost godfather Sirius Black. But he wasn't the same Sirius Black Harry remembered. The man had been brainwashed by a woman known as 'Mother Miranda', who used an ancient entity known as the Megamycete to control a massive cult of werewolves. Harry was forced into the cult unwillingly, before being seduced by Miranda as well as her adopted daughter Alcina Dimitrescu. At the end of his journey, Harry would be murdered by a mysterious Grim Reaper. The mysterious stranger would then lead Harry into an unknown fate, away from his new life.
Book of Shadows Pt. II: The Upside Down:
After Harry Potter's death at the hands of a Grim Reaper, he was sent into a realm of the Multiverse belonging to an entity known as The Watcher. Once there, The Watcher described Harry's negative effects on the universes he arrived in, explaining that in all of the universes he visited that weren't his own, Harry's appearance marked the beginning of an infection. A plague destined to destroy a universe. After a brief battle between the two, Harry crashlanded on the surface of a new universe, now stuck in the malnourished version of his body.
Once more, he was captured in the immediate moments after his arrival, this time by the scientists of Hawkins National Laboratory. Upon reaching the Laboratory, Harry met the famous doctor, Dr. Martin Brenner. Harry escaped the doctor as well as the Laboratory and went on to meet Johnathan Byers, Nancy Wheeler, Joyce Byers, and Jim Hopper in the Hawkins Police Department. He offered his assistance to the troubled group - who were facing a monster from another dimension - and helped find the youngest members of the 'Party'. Mike Wheeler, Lucas Sinclair, Dustin Henderson, Max Mayfield, and Jane "Eleven" Ives. After assisting the group in killing the dimensional creature - known as The Shadow - Harry Potter would go on to lead the magical world out of obscurity, gaining abilities in soul manipulation along the way.
After marrying Hermione Granger and Daphne Greengrass, the world was plunged into an invasion unlike anything Harry had ever seen, an invasion led by the Voldemort of his home universe. In the span of six months, the war took a turn for the worst, resulting in most of Earth's population being evacuated off of the planet. With the planet in the control of Voldemort, Harry and his forces were forced to launch a campaign through the Multiverse to stop and kill Voldemort before he ever invaded the planet. On their way to his home universe, the group was split into three groups. Harry returned to Raccoon City in the universe he had left before, Eleven traveled to the future of an alternate universe, and Alyssa Truit - a doctor within Harry's company - traveled into the distant future. The three groups eventually reunited - meeting Valentine Wiggin along the way - before going on with their mission to kill Voldemort. They were successful and returned to their home universe to find that time had altered so that the invasions had never occurred.
In the aftermath, Harry married Eleven and Valentine and continued his work in saving humanity. In the concluding chapter of Book II, Harry discovers that the universe has shifted once more, resulting in the presence of Raccoon City. After the death of his wife and unborn child at the hands of a former antagonist, Harry Potter decides to reset the Multiverse by creating the perfect universe. Yet not everything seemed to go the way he wanted it to.