Trial by seven

"Once in a long while, you may get two real highborn shits having a go at each other and one of them might be fool enough to demand a trial by seven. Exactly how it sounds: seven men against seven men."

- Bronn

A trial by seven is a variation of a trial by combat. Very rarely, after the accused has demanded a trial by combat, he may also demand a "trial by seven": instead of one man versus one man, two teams of seven men each will fight.

Rules
As with a normal trial by combat, the accused and accuser each have to pick six other champions - though each also has the option to not fight in person but to name a seventh man as their personal champion. A trial by seven ends only when all seven men on one side have been defeated (either by yielding or dying).

Known trials by seven

 * Trial by seven of Duncan the Tall

History
While attending a tournament at Ashford in the Reach, Ser Duncan the Tall, a poor hedge knight, became smitten with a beautiful Dornish woman named Tanselle. However, the arrogant Prince Aerion "Brightflame" Targaryen accosted the young woman, at which point Ser Duncan sprang to her defense - as any true knight should - and struck Prince Aerion. To strike a member of the royal blood was treason, and the corresponding sentence would usually be the loss of the offending hand. Ser Duncan, however demanded a trial by combat, which, at the insistence of Prince Aerion, then became a rare "trial of seven", involving two teams of seven men each.

Ser Duncan's side was victorious, but during the combat Aerion's uncle Crown Prince Baelor "Breakspear" Targaryen accidentally took a severe head injury, and the resulting concussion killed him.

In the books
In the A Song of Ice and Fire novels, the trial of seven is a form of trial by combat an offended party can demand during trial. It is linked to the Faith of the Seven and Andal tradition. 

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. There has not been a trial by seven in almost a hundred years; the last known use of this procedure was during the Ashford Tourney in year 209 AC. It was initiated after a dispute between Prince Aerion Targaryen and Ser Duncan the Tall.