Harry James Potter, The Boy Who Lived, The Man who Conquered, and The Chosen One was attacked a little after the battle by one of Voldemort's followers. The woman had grabbed Harry when no one was looking and tried to cut his throat. Fortunately, she failed to kill him but unfortunately, Harry would never speak again. This would cause him to flee the wizarding world.
Harry had after the cab driver to take him to the airport when Harry's second accident happened. A truck barreled right towards the cab. Harry barely had enough time to grab his wand before everything went black.
During this time glamours and binds on Harry broke changing his looks enough that if he went no one would recognize him. His wand broke and the magic of the wand would be set in his hand for the rest of his life. Perhaps the biggest change was his memory as Harry Potter died from his memory disappearing a new man was born.
When this man woke up in a hospital the only thing he could truly remember was he was 18 and October 31st was important to him. The guess was that that day was his birthday.
When the man signed his name it looked like Harwyn Eight Liam to most who read it. Thus he left the hospital not knowing who he had been but who he was now.