A Knight of the Seven Kingdoms episode 4 explained: What is a trial of 7?
With only two episodes remaining, the fighting is finally kicking off. Egg's true identity has been revealed, and Dunk is in serious trouble for beating Prince Aerion after doing what he thought was right. The trial of seven is set to begin in episode 5. In episode 4, Dunk awaits punishment and Baelor Targaryen tells him he has a right to a trial rather than immediate imprisonment or death.
Dunk asks for a trial by combat, and Aerion accepts on the condition it be a "trial by seven." Baelor explains the rite came across the Narrow Sea with the Andals, who believed that seven champions would honor the gods and invite divine intervention. Dunk has until dawn to find six allies or he forfeits the trial.
Daeron, Egg and Aerion's brother often called Daeron the Drunken, is central to the dispute; his drunken sleep at the inn prompted Egg to pose as a stable boy and become Dunk's squire. In the book, Daeron has prophetic dragon dreams, a gift tied to dragon blood.