William was the illegitimate son of Duke Robert of Normandy. He became Duke of Normandy after his father’s death. He claimed he was promised the throne of England by Edward the Confessor and also claimed that Harold Godwinson had promised to support his claim to the throne. When Harold Godwinson was crowned King Harold II after the death of Edward the Confessor William planned an invasion. William won the Battle of Hastings in October 1066 and was crowned King of England on 25th December 1066.
Family tree for William I showing: