|"Is this all real? Or has this been happening inside my head?"
The topic of this article is of a real-life subject that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really exists), and, as such, details contained in this article may differ from real world facts.
He led the Norman conquest of England in 1066. The wizard Armand Malfoy was among his invading army. Sometime after being crowned king, William granted Malfoy a piece of land in Wiltshire, in return for questionable services which Armand had rendered him.