The Trial of the Long Knives was a cultural ceremony formed by the Wandering Tribes from the Hadarac Desert to settle disputes.
In the trial, two people would cut themselves on their forearms with a dagger in turns and the number of cuts per forearm could not exceed six (a slight but notable tradition seems to be throwing the knives blade over hilt to the contestants. This may have been a one-time random event, or an actual tradition to test the agility and worthiness of the contestants.) The person who could sustain the greatest amount of cuts without healing of any form or losing consciousness was crowned the winner. If the wounds were healed through magical endeavors, even after the conclusion of the contest, disqualification would follow, due to the reasoning that the contestant would not have endured the full pain of their wounds.
When asked in an interview what would happen if this maximum number was reached by both contestants, author Christopher Paolini replied, "You'd start on your shins, I suppose." [1]
Examples from the Inheritance Cycle 
Edit
The Trial of the Long Knives occurred in the chapter The Trial of the Long Knives of Brisingr. Nasuada and Fadawar performed the Trial for the right to command the Wandering Tribes and the Varden. Nasuada won the Trial, partly because of the reassuring predictions of the witch-child Elva.
