The current true goal and purpose of Shuukaku is unknown, although due to the behaviour of the members of the cult it can be speculated that the end goal is having members achieve continuous improvement to the point of perfection, attained by repeating testing of one's abilities against ever-harder enemies in ever-tougher circumstances.
Little is known about the base of the Shuukaku cult, although it is known that it is an underground cave (or a network of caverns). It is also known that there are various crafted objects (such as a tent, as well as clothing and some ceremonial weaponry) there that have been obtained through unknown means.
The Shuukaku cult has five levels of seniority (with an additional, special circumstances one). The titles are as follows:
Acolyte (special circumstances)
Acolyte: Acolytes are cultist members that have been recruited on the surface world and that haven't had any formal training in the cult, but were instead only acknowledged by a single Berserker or higher ranked member. Acolytes can transition to become true members and attain Berserker rank by gaining the trust and respect of a Berserker cultist in addition to proving their strength in combat. Once those criteria are met, they may naturally progress to Berserker without the need of becoming a Disciple first.
Disciple: Disciples are considered to have started their training from as early as newborn babies and are always under the age of 25. Disciples spend all of their time undergoing extremely intensive training during every waking moment of their lives. They are grouped in pairs and put under the direct supervision of a cult member ranked Harvester or higher. Additionally, Disciples might be taught a specific sub-doctrine propagated by their specific mentor, such as "The School of Destruction" or "The School of Silence" etc.
Throughout their training the disciples undergo intense physical exercises as well as daily combat training and extremely violent "spars" that often times might even lead to a gruesome death. They are also regularly forced to kill captives in order to get used to murder. Additionally, a lot of time is spent in teaching them the philosophy of the cult and their chants.
Disciples are required to pass through multiple, deadly, varying trials with the final and hardest one being the "Trial Of Feats" that involves two paired Disciples who've spent their life fighting together to lock into a deadly duel and the one who emerges victorious fights a human captive from the surface world (as strong as possibly available). If the captive is killed successfully, then the "Trial Of Feats" is considered successfully completed and the Disciple is now recognized as a Berserker, earning the right to roam the surface world (by "replacing" the killed captive) and begin their "Pilgrimage Of Blood."
Berserker: Only the most renowned Disciples reach the prestigious rank of a Berserker and begin their "Pilgrimage of Blood". The Berserkers spend their time dwelling on the surface and looking for constant self-improvement in terms of their combat skills, endlessly seeking stronger and stronger enemies to face off against.
While retaining the status of a Berserker, cult members are almost completely independent, only bound to the rule of taking care for their fellow cultists and following an Elder's orders in a time of need. During the time spent on the surface, a Berserker's sole duty is to strive to complete their "Pilgrimage of Blood", but another important reason to leave for the surface is to enrich themselves with new knowledge and experience new things, as well as adopt new points of view that can hopefully strengthen the cult further.
The overwhelming majority of Harvesters gain their unique personality traits from the surface world and usually take the item they found most significant to them back to the caverns as a memento. Every sub-dogma has been derived from the experiences of a Harvester's experiences as a Berserker. Due to their intense training, a fairly high amount of Berserkers manage to make it past their "Pilgrimage of Blood" and reach the next rank in the cult.
Harvester: Harvesters are accomplished cult members that have completed their "Pilgrimage of Blood" and have been officially recognized as proficient killers within Shuukaku. This is generally the highest level that the average successful cultist can reach.
Upon returning from the surface world, they settle in the caverns again, where they continue honing their (now supplemented by new experiences) craft - the art of killing. Harvesters have two purposes - one, to fight for the cult when called upon and two, to mentor Disciples and shape them into excellent cultists. Widely recognized Harvesters can even create their own sub-dogma to teach their pupils.
Harvesters often wear a uniform black robes in addition to some masks and usually have a mastery over at least one type of weapon. A Harvester can be promoted to an Elder if they produce a high quantity and quality of successful Berserkers.
Elder: Once a Harvester becomes an Elder, they become a part of the so-called "inner circle" of Shuukaku, gaining immense influence within the cult. This inner circle can never go above 13 in number (12 Elders and a Seer). An Elder's duties are not clearly defined or known, but it is known that they write the history of the cult and that they participate in most rituals/ceremonies. Sometimes Elders might also take upon themselves to train Disciples, but that'd be because of their own will rather than duty. They are also the ones to supervise the operations of the cult and command essentially everything and everyone.
Seer: The criteria for becoming a Seer are unknown and the exact duties of a Seer are also not clear. All that is known is that the Seer is the absolute leader of Shuukaku, having complete control even over the Elders and supreme authority. It appears that the Seer has the lead role in ceremonies and rituals, but in addition to that little is know about the position.
The members of the cult are quite open about being extremely violent and savage. It appears that in their culture, life means very little and is significantly redistributed among members as they see fit. It also appears that it is considered an absolute must to show respect to equal or higher ranked cult members. Cultists seem to follow some sets of rituals as well as the cult's internal rules. When it comes to relationships between mentors and students, everything is left to the mentor's judgement - some mentors may treat their underlings kindly while others might be excessively brutal.
There are also some "tests" that the cultists are put through:
"Trial Of Feats" - The "Trial Of Feats" consists in having two Disciples that have been together all their lives fight in a deathmatch for the chance to duel a surface dweller and steal their place if they win, earning themselves the rank of Berserkers.
"Pilgrimage of Blood" - The "Pilgrimage of Blood" is the trial that Berserkers undergo in order to becomes Harvesters. It consists in the Berserker starting their life in the surface world. And killing an unspecified as of yet amount of opponents before they can return to their home back in the caverns.
Shortly after its formation, they became a part of Aogiri Tree and have thus mostly conformed to the orders and ideals of the group. In-fact, the goals of the small cell are unclear, save for their insatiable appetite and desire to grow more powerful.
After the destruction of Aogiri's 11th ward base, the remaining four took up residence in Haruna Suzuki's 11th ward Archive for but a short while. It is currently unknown where the cultists reside.