King Mark was searching for ways how to strengthen his position in Arthur’s empire – searching allies in Kay, Morgause, and Morgana. 

He got in touch with the Majordomus Kay who identified Guinevere as a person who has the key influence on Supreme King Arthur. 

Morgause introduced people to Gwenhwyfach – Guinevere’s half-sister and doppelganger. But it was Morgana who come up with the final plan. She used Dinadan to instruct Gwenhwyfach about Arthur. Morgause poisoned  Guinevere’s father Leodegrance since he was the only one who could prove which girl is the real Guinevere. Then, Morgana sent her servant Aoife to Arthur with a message claiming that Arthur’s wife is a fraud. It said that Leodegrance loved his illegitimate daughter about all and switched her at the wedding night. 

Arthur traveled with Dinadan to Gwenhwyfach’s castle and was immediately enchanted by her. To strengthen the girl’s self-confidence, Morgana prepared a strong aphrodisiac, claiming it is a Love Potion. Gwenhwyfach gave it to Arthur and herself and spend a night full of passion with him. 

Arthur took  Gwenhwyfach back to court and prepared a public test of the “true Guinevere”.  He prepared a series of questions a real loving wife should know. 

Both Kay and Dinadan had access to the questions and gave them do Gwenhwyfach in advance, so during the test, Gwenhwyfach shined, and Guinevere failed miserably. 

Guinevere was sentenced to burn at stake for treason, but Lancelot (supported by Bors, Balin Ill-Fated, and Hector) charged the execution, saved her, and killed two of Morgause’s sons (Gawain’s brothers) during that. They fled to Balin Ill-Fates’s castle. 

The siege was going on for two years. Lancelot was supported by his friend King Galeholt. In the meantime, Morgana and Mordred came to the court and became King Arthur’s advisors. Both King Mark and Majordomus Kay had a great influence as well. 

But then,  Gwenhwyfach felt sick and died soon after. You don’t know if she said to Arthur anything and he never caught any of the conspirators, but he accepted Guinevere and all rebels back. As a result, the conspirators’ power at court is still big, but smaller than before.