Each time Harry Potter dies he wakes up in a new body, usually of someone recently deceased, and by recently, I mean at the moment of their death. As the soul of the person who originally inhabited the body leaves, it passes through Harry's soul, leaving him with an impression of all that persons memories up until the moment of their death.
Since souls exist outside of the constraints of time, there is no telling when he will be placed into a new body, nor who it will be, or if it will even be in the same dimension he originally inhabited, not that there is much difference between them.
Unbeknownst to Harry, he is actually being placed into the bodies of the "Potentials" or people who stood to become great catalysts of change or people who are essential for tipping the balance one way or another. These individuals, being so close to fate, are often also too close to death, and occasionally the vicinity to death accidentally leads to their early demise before they can fulfill their fated purpose.
Master of Death is not really a defined role, and in truth, was not one ever meant to exist, but the universe does love to fuck with Harry, so when he accidentally did the impossible, "Death" had no idea what to do with him, so along with the other Incarnations, they came up with a plan to use him and his natural "Potterness" to have a bit of fun with the humans, they do get so very bored of the same thing, over and over again.
Harry has lived through dozens of lives by now, his own original one ended at the age of thirty-two, languishing in a job he hated, married to a wife who hated him, lauded as the next Merlin by some, and the next dark Lord by others, his best friends all grown up and living lives of their own with jobs and kids and seemed happy in a way Harry wasn't certain he even knew how to be.
He was killed by a muggle bullet to the head of all things, while investigating a human trafficking ring.
He woke up moments later as a 12 year old girl in the 1920's who had just died of Influenza. He lived as Anna Maria Fozzarelli for the next two decades, and played an instrumental role behind the scenes in deposing Benito Mussolini and ending his fascist regime, though she did not get to celebrate as she was murdered by her half brother just two days after Mussolini's execution, ostensibly so he could inherit a small property left to her by her birth mother's father to sell so he could pay off a few gambling debts.
Harry's next life was as a legionaries in ancient Rome. He lasted about a decade as Caius Polunius fighting a secret war with an alien race that had been masquerading as Gods in an attempt to enslave humanity. He was instrumental in tipping the balance, and ultimately gave his life in willing sacrifice, giving his comrades the means to seal the aliens away in an alternate dimension.
His next life was pretty tame, he was the quiet, supportive wife of a theoretical scientist in the late 2300's. He spent fifty years playing housewife and assistant, to the man who ultimately designed a way to create a contained earth like ecosystem with a breathable atmosphere and functional ozone layer for a terraformed planet, solving earths overpopulation problem. Harry died due to a cryosleep malfunction en route to their new home on Mars.
And so he went on, life to life to life. Each one lived, including his original one existing as distant memories, his current life taking precedence at the forefront of his consciousness, the only thing that is different from the original individual he is replacing is magic, as that is tied to ones soul. Magic which manifests in different ways in each new life he lives.
His current life, he can feel it coming to an end, something that has only happened twice so far, as his death is usually sudden, and brutal.
The first time he could feel it ending was when he'd been reborn as a child with fourth stage brain cancer. He lasted nearly eighteen painful months, his time in that dimension spent inspiring his grieving scientist mother to create a cure for cancer, a cure that came too late late for him, but changed the lives of millions.
The second time he had felt the slow approach of death was the only time he'd ever lived an entire lifetime to eventually die of old age. That life was truly an anomaly, as he'd been reborn just moments after his host body had been born. He lived to the age of 86, and was a musician, first in the court of King Henry the VIII, and then in the courts of the man's three children. He never married, though he'd had several liaisons with influential historical figures of the time. He'd even shared a daliance with Anne of Cleaves, after her marriage to the king had been annulled.
In stark contrast, this life was a short one, and from the moment he'd been reborn, he knew he wasn't long for the world. His host body was that if a sixty year old woman, a long term resident of a prison planet, her crime being born female to the wrong family. In her first month in this life, she'd managed to lead her fellow prisoners in a rebellion against their jailors, freeing them and taking the planet for their own. Her second month though, marked a plague that had been released in retaliation against the now freed prisoners. And this, her third month had marked them stealing a starship, which they was currently using to escape the planet with as many still left alive as they could fit, to search for a cure.
She died before they could find it, but only after she realized the plague was so contagious that every being they crossed paths with contracted the virus, no matter the species they were, which made sense, since the prison planet hosted such a wide variety itself.
As she lay there, breathing her last, she came to the sudden realization that she'd probably just caused the death of likely millions.
*OH, MUCH MORE THAN THAT, LITTLE MASTER. YOUR ESCAPE HAS TRIGGERED THE END OF ALL LIFE THROUGHOUT THAT ENTIRE DIMENSION. WITHIN A CENTURY AS YOU ONCE MEASURED ON YOUR ORIGINAL EARTH, THERE WILL BE NO MORE LIVING ORGANISMS LEFT. CONGRATULATIONS! YOU HAVE FAR EXCEEDED OR EXPECTATIONS, SO MUCH SO, WE HAVE DECIDED THAT YOU HAVE EARNED A REWARD. ENJOY!"
Wait, what?
And just like that, he awoke in his original body. Well, a very young version of his original body. If he had to guess, he was at least eight, and definitely not yet eleven, since he was laying on the crib mattress in the cupboard under the stairs.