1. How did Modred become king?

sorry this was like last year but im going to answer it any ways

The unhistorical account presented by Geoffrey describes Arthur leaving Mordred in charge of his throne as he crossed the English Channel to wage war on Lucius Tiberius of Rome. During Arthur's absence, Mordred crowns himself king and lives in an adulterous union with Arthur's wife, Guinevere (Ganhumara).