Trial of seven
The Andals believed that if seven champions fought on each side, the gods thus honored would be more likely to see justice done. If a man cannot find six others to stand with him, then he is obviously guilty.
In 42 AC during the Faith Militant uprising, Ser Damon Morrigen of the Warrior's Sons challenged King Maegor to a Trial of seven to which King Maegor accepted. Ser Damon and six Warrior Sons fought against the King and his six champions. It was a trial in which the Kingdom itself was at stake. In the end only King Maegor was left alive, the other thirteen lay dead. 
The trial of seven is rarely invoked in modern Westeros. Prior to Ashford There had not been a trial of seven in over a hundred years.