In the unknown year when the Aogiri Tree was founded, the future soldiers of the Aogiri Tree were just numerous regular ghouls who had ran into the preachings and ideology of ghoul supremacy by the mysterious One-Eyed King. Many ghouls had been attracted to the king and so they joined the newly formed Aogiri Tree believing that ghouls were the dominant species and that humanity must be overthrown.
Search for Rize
Years later in present day Japan, the Aogiri Tree is on the hunt for the mysterious ghoul Rize Kamishiro and seek her for a purpose unknown. Aogiri soldiers at this time, had engaged the CCG forces in places like the 11th Ward and have successfully taken over the entire ward and have made it their headquarters. Aogiri troops are also placed under the command of people such as Yakumo Oomori and Ayato Kirishima as well. While they don't find Rize (unaware of the Steel Beam Incident), they do find Kaneki and he's brought back to the 11th Ward Base for havinf Rize's stench.
11th Ward Battle
On December 29, the forces of the C.C.G. after discovering the Aogiri Tree, had sent a large detachment to the 11th Ward Base where the Aogiri Tree and CCG would engage in battle. The flare was shot up and soon Aogiri and CCG soldiers began to attack and fire upon each other. At first, the Aogiri Tree forces had the upper hand with the CCG pinned down, but investigator Juuzou Suzuya stepped in and turned the tides, whipped out the Aogiri forces outside, and allowed the CCG to storm the compound. For the rest of the raid, CCG and Aogiri Tree soldiers would engage in close quarters combat inside the base and had to escape when the base was destroyed but the CCG had won in the end.
Second Cochlea Raid
Sometim during the 11th Ward Battle, Eto Yoshimura and Tatara rally a bach of Aogiri soldiers and they all attacked Cochlea to free the ghouls there and recruit them into their armies. The Aogiri forces attacked and soon quickly overran the facility and broke out all of the ghoul prisoners. The CCG had arrived with reinforcements as the warden was killed and numerous security officers were as well but it was a major victory for Aogiri as they were able to acquire new soldiers for the organization. One of the new recruits was Matasaka Kamishiro who joined the Aogiri Tree by the orders of Yoshimura in secret.
In the 20th Ward, the CCG had discovered the location of Anteiku, a cafe that was a secret ghoul organization and sent in their forces to destroy it. Anteiku was soon backed up by the Black Dobers and the Apes who gathered one last time to assist Anteiku where their former leaders were members of. Over the course of the long and grueling battle, the CCG suffered heavy casualties but won, only to have Eto come in and attack them. Things would only get worse as legions of Aogiri soldiers descended and attacked the remaining CCG forces. At first Aogiri was winning, when Kishou Arima had showed up and brought with him reinforcements. In the bloody aftermath, Aogiri had been forced to retreat due to Arima's arrival and his wounding and near-defeat of Eto.
Two years later, the Aogiri Tree remains active and has now expanded to have more territory and new wards. One night, Aogiri forces are hired to act as security for an upcoming auction where ghouls will bid and buy humans for their own personal gains. These ghouls are high and upper class ghouls and Aogiri troops will be their to act as security. The CCG bust in however and the Aogiri troops there engage them in combat. Despite taking down numerous ghoul investigators, the Aogiri soldiers suffer heavy losses and the remaining members are forced to retreat.
Rushima Landing Operation
After some time has passed and conflict continued to rage on, the Aogiri Tree locates to Rushima Island, a former military base, as their new headquarters after being forced out of the 11th Ward. Eto is there and hosts a meeting with known prominent members of the Aogiri Tree and the organization's soldiers. Eto states how she's not the king and that Aogiri has suffered some losses but is still strong. She eventually leaves and is captured by the CCG after turning herself in making it her final meeting. Eventually however, the CCG finds out that the Aogiri Tree is stationed there and sends in an army to exterminate them. Aogiri makes one final stand as their soldiers engage the CCG troops in bloody combat. Despite a long and gruling battle with countless CCG forces taken down, the Aogiri soldiers are defeated with 98% of all remaining Aogiri members killed. Survivng Aogiri members are seen escaping off of Rushima, but the members are all known named members and are officers. No Aogiri soldier is confirmed to have survived making most of them dead as they fall with the Aogiri Tree as it's dissolved.
Aogiri Ghouls wear large dark red robes with skull-like masks with red eyes and their bodies are completely covered by their robes. In :re, they have different masks with a white coloring over a blak lining over the eyes and the center of the face.
Members and soldiers of the Aogiri Tree are hardline ghoul supremacists who have sworn absolute allegiance to the One-Eyed King. They believe that ghouls are the dominate species and will make sure that it becomes a reality and that humanity is overthrown. They use all of their abilities to their advantage and the hardliners often go to great extents.