This lore can be found by a Bounty Hunter on Taris.
Translated directly from the original Mando’a, Geroya be Haran literally means “game of annihilation”–though the challenge is anything but a game. While Mandalorians often engage in lethal duels to settle matters of honor, death in such a contest does no injury to a Mandalorian’s legacy. To be challenged and to lose the game of annihilation isn’t just to die–it is to have every accomplishment, every honor and every memory wiped away as if none had ever existed. Geroya be Haran is the gravest challenge a Mandalorian can issue. The Geroya be Haran is so rarely invoked that the tradition’s origin has been lost to time. However, many believe that it is the symbolic retelling of one of the Mandalorian Neo-Crusaders’ most absolute victories during the Mandalorian Wars.