Arthur was born the Son of Uther Pendragon and Duchess Igraine. They weren't married so he was considered a mistake. Later on his father gave him away to Merlin. Who later gave Arthur to Sir Ector.
Arthur goes to his brothers tournament and forgets the sword. He goes looking for a sword and accidentally pulls out Excalibur.
Arthur became the king of Britain and established the round table. He was also married to his wife, Gwenevere.
Later on he found out his wife was cheating on him with his best friend. He demanded for Gwenevere to be hung/public execution.
Lancelot disobeyed Arthur's words and saves Gwenevere from her death. This causes a battle between Arthur and Lancelot.
Arthur dies in battle between his knights and those led by Lancelot.