Final answer:
Guinevere fell in love with Sir Lancelot, which caused the downfall of Camelot.
Step-by-step explanation:
Sir Lancelot is the knight that Guinevere fell in love with and his affair with her ultimately led to the downfall of Camelot. In the legend of King Arthur, Sir Lancelot and Guinevere's forbidden love caused turmoil and conflict within the kingdom. This betrayal of King Arthur's trust resulted in the downfall of Camelot.
Learn more about King Arthur and Camelot