Final answer:
In the legends of King Arthur, it is usually knights who tell Arthur of Guinevere's infidelity with Lancelot, as depicted in 'Le Morte d'Arthur' by Sir Thomas Malory.
Step-by-step explanation:
In the legends of King Arthur, the affair between Guinevere and Lancelot is a well-known story. In many versions of the legend, it is usually one or more knights who tell Arthur about the infidelity. For example, in Sir Thomas Malory's 'Le Morte d'Arthur,' it is Sir Agravaine and Sir Mordred who tell Arthur of the affair.
They reveal the truth to Arthur out of their own personal grievances and a sense of duty to uphold the ideals of chivalry.