In the reign of Fire Lord Mitzuo, grandfather of Sozin, a new guard was created, the Kaji, as a Fire Nation Dai Li only more covert with a far more direct goal and no go to from an Avatar. This guards sole role was to protect the Fire Lord from all outside harm, other nations, or their own people. Any threat to the Fire Lord had to be neutralized even if they themselves were a threat. Many of the guards were forced to take their own lives and the lives of people close to them for the Fire Lord, because they were compromised and none must know that they exist. The guards were chosen from previous guards' families or benders who were closely related to the Fire Lord, for family did not attack family. Siblings of the wives of Fire Lords were chosen as no Fire Lord had any siblings, as to disallow competition for the throne resulting in no heirs.
The Kaji were trained starting from birth in the arts of espionage, sabotage, assassination, tactics, disguises, poisons, explosions, interrogation, torture, firebending, chi-blocking, spirit preventing, and combat. Each member was expected to know how to use anything around them as a weapon to kill from food, to paper, to ordinary objects. Only 50 could exist at the same time and all Kaji were members until death, older trained younger and the cycle went on. All Kaji wore black masks with red outlines from the forehead to below the eyes in the shape of curling flames. They wore black hoods to cover the rest of their heads, black clothes, and cloaks hiding a variety of weapons including wires, poisons, shirshu darts, exploding gel canisters, and various other sharp objects.
Wherever the Fire Lord was at least four Kaji were watching, the other 46 were out stopping plots against the Fire Lord, assassinating threats, and discovering delicate knowledge from the other nations. Many of the threats were stopped with a quick poison or shirshu dart to the neck without anyone being any the wiser. Only the Fire Lord was told of their existence, not even the heir was to know until told by the previous Fire Lord at their death. When Kaji died, they were honored by their members then to all the rest of the world their membership to the Kaji was destroyed.
The Kaji worked in groups of four called Zai, one for each of the four nations, one to neutralize each. The remaining two members were the best-of-the-best and one worked alone in other nations or protecting the Fire Lord on important days with a Zai and the other was the leader called Phoenix. Zais were not allowed to know of each other's identities and only called each other by single names of animals such as, komodo or ostrich. There were 50 names used over and over.
When Fire Lord Sozin began his conquer ship of other nations, the Kaji were at odds at what to do, help the war effort as many were instructed, out of their personas, or continue being a Kaji and look like traitors to the Fire Lord. Phoenix decided that any drafted into the war effort would become soldiers but discover all they could in relation to plots against the Fire Lord and Fire Nation and destroy them.
Sozin began creating colonies in the Earth Kingdom with armed soldiers and the soldiers' families. Sozin expected Earth to fall just as easily but Earth was stubborn and steadfast and they resisted. When Roku attacked Sozin the Kaji had been order to allow it to happen, Sozin wanted to try his strength against an Avatar. When he lost a Zai patched him back up and gave their reports on Roku's weaknesses. His main one was his friendship with Sozin, he would not kill him for the sake of it.
When the volcano that was Roku's home island was erupting, the Kaji told Sozin where Roku was trying to stop the lava flow with his dragon Fang. Sozin helped Roku because even though they were enemies the people of the Fire Nation came before any conflict. When Roku succumbed to the poisonous gases Sozin left, for he could do as he wished against a baby Avatar. He began planning and plotting to destroy the Air Nomads. The he discovered the existence of a comet that would increase a firebender's strength to the power of a hundred suns, thus giving firebenders the strength necessary to destroy the Air Nomads.
The Kaji joined Sozin in all the battles against the Air Nomads, leaving only few alive for interrogation and torture. The Kaji specifically tortured Master Gyatso, the Avatar's teacher, for information. All they found out is the Avatar had left on his sky bison before the Fire Nation attacked. Then Gyatso killed every Fire Nation soldier around him before dying himself. The Air Nomads were destroyed and the true conquest of the Fire Nation had begun.
The Earth Kingdom falls slower than the Air Nomads, they already knew that the Fire Nation would attack. Earth was far stubborner and steadfast than Air. After Sozin died his son Azulon continued his work. Azulon continued the conquest of the Earth Kingdom taking more of the western side.
When Azulon's son Iroh was born, great rejoicing was found in the Fire Nation, but when Ozai was born only confusion. Why did the Fire Nation need a second heir, they already had one another would just cause conflict? When Ozai was old enough to become a Kaji there was already 50 members and the Zai that saw Ozai found something off about him. He seemed to have no loyalty to his brother or father so he was immediately disregarded for the Kaji.
