Chronicle of Ages
The Mantis is the only Great Clan that was not founded by one of the Kami that fell at the Dawn of the Empire. Instead they spent the majority of the Empire’s history as one of the oldest Minor Clans. That all changed on the Second Day of Thunder when they were elevated to a Great Clan.
The original territory of the Mantis were the Islands of Spice and Silk. As a result the clan developed a mercantile and sea faring culture. This meant that they often were the first to encounter gaijin. This resulted in their eventual mandate of protecting and patrolling the lands that do not belong to any other clan, especially the seas.