Twelve Guardian Ninja of Fire (
Official Version)
This was a small group of elite shinobi that served and protected the Daimyo of the Land of Fire as his personal bodyguards, permanently living in the imperial palace with the royal family. They would constantly remain by the Daimyo's side to provide protection from any enemy or misfortune, no matter where they might be.
Most of the Guardians were descended from the samurai clans that served as the daimyo's vassals During the Warring States Era where shinobi gained prominence and became the strongest military force, these samurai adapted to the changing times by becoming ninja themselves and learning to art of ninjutsu to better protect their lord. Other Guardians were chakra-wielding warriors or ninja who saw no greatest honour than dedicating their lives to protecting their country's leader.
Even after the Hidden Village Era began, these Guardian Ninja remained as the Daimyo's greatest protectors. Not loyal to any established ninja village, it provided the Daimyo with a check against any Hokage that might decide to turn against him and a stalwart defence against assassins and kidnappers.
It wasn't until several decades had passed that an allowance was made. To deepen the relationships between the Daimyo and the ninja factions of his country, he sent an invitation for Sarutobi of the Hidden Leaf and Chiriku of the Fire Temple to join his ranks.
But this same Daimyo had the idea of using his bodyguards for personal missions that allowed him to act independent from Konoha despite them being in charge of the country's military affairs. In many ways it was because of Konoha's weakness after the Third Great Shinobi War and the following incidents that fostered the idea in his mind, believing that the village wasn't strong enough to protect him and his interests as well as it used to.
With the Daimyo directly commanding his own private shinobi force, the belief that he should rule the Land of Fire alone gained momentum. To the Guardians who thought that their country only needs one ruler, the Hokage seems an unnecessary role that stole power from the man who should be in charge. Only not all of them agreed with that or felt that being used for the Daimyo's personal missions was the right thing to do. Tension built within the group that boil over into an attempted revolution that started a civil war within their ranks. The coup d'etat was a failure and in fact ended in the death of the current Daimyo and several of the Guardians.
His son came to power as the new Daimyo who had no desire to dictate the military matters and was quite happy to leave bloodier affairs to the Hokage. To better the relationship between the country and its hidden village, he disbanded the Twelve Guardian Ninja to remove the problem at its root and then entrusted Konoha's shinobi with his protection.
Some say it was the leader of the rogue faction killed him with the hopes of installing a puppet daimyo and ruling the Fire Nation from the shadows. Others whisper that the previous daimyo supported the attempted coup-d'etat because he wanted sole control of the Fire Nation, and the Third Hokage had him assassinated in retribution. There are even rumors that the bounties on the heads of the surviving former members of the Twelve were placed there by wealthy supporters of the previous daimyo who blame them for his death.
Asuma, Chiriku and the other surviving Guardians returned to their homes, carrying back with them elite skills, monetary rewards and a great amount of prestige.