The Black Robes
Originally founded in 2068 on Earth, this highly organized group obtained "cult" status in 2083 from very stupid and ignorant people. They where in fact, before they split into multiple Sanctuaries across the galaxy, honourable assassins, with Japanese and Western European cultural origins, heavily revelling in chivalry, knighthood, respect, and honour. After the First Contact War, with the recruitment of multiple aliens with different cultures of their own and the Black Robes spreading across the galaxy, the Black Robes were split into multiple Sanctuaries and Splinter Groups, who more or less followed the original traditions. Grand Clerics are the Sanctuary leaders and are very powerful. They also provide contracts as middle-men for the lower ranks and help organise far more complex assassinations.
Before the Cerberus op to destroy all Black Robe Sanctuaries, there were 27 different and unique Sanctuaries across the galaxy. They all have adapted their culture and habits based on their Grand Cleric and their preferences. Sanctuaries are basically hideouts, that consist of 3-10 members. Sanctuaries are usually equipped with all the necessary rooms and chambers, ranging from barracks to torture rooms, depending on the Sanctuary and their Grand Cleric.
A Splinter Sanctuary that consisted of a mix of 36 mass murderers and the criminally insane of all kinds of races, they completely threw traditions and combat techniques aside. Despite this, they were a very deadly group of individuals who were frowned and hated upon by the more respectable Sanctuaries. Their Grand Cleric was a genocidal murderer who enjoyed torturing the weak and mutilating them, which could be the reason why he split from the Black Robes of old and only recruited insane and satanic people into the ranks, he also made himself somewhat of a cult leader. The way they completed their contracts gave the Black Robes a very bad image.
Based on Illium, hence the name, this Sanctuary consisted of 42 Asari Biotic-Sisters and 2 Turian Biotic-Sisters. A female only Sanctuary, their biotic techniques were a marvel in Black Robe martial arts, replacing the traditional pistols of the Gun Clerics' Gun Kata with biotic abilities, mostly consisting of Warp and Shockwave, and in more complex move sequences, an occasional Singularity. They also still followed the Black Robes traditions of the Old Earth to the letter, making then chivalrous and respectful of their opponents, which was odd for a group of Asari. They were well respected, and since they made biotics a total weapon (They only used biotics and hand-to-hand, no weapons at all), their Biotic Clerics could be compared to Asari Commandos. They were always called upon by the Dawnstar Sanctuary for help with more complex contracts. Their Grand Cleric was a very powerful biotic, but also quite young among her companions. It is believed that she survived the Cerberus attack.
The original Black Robes of Old Earth, they were based on Earth, in the rural country side of former England. Consisting of 73 members, their Santuary was an underground fortress, which was an old retro-fitted military nuclear fallout bunker. The followed their original traditions to the letter, and were the founding fathers of the Black Robes. Their Clerics where heavy practitioners of Gun Kata. Making them very deadly opponents and never to be taken lightly. Their targets were to be killed in a traditional manner of honourable hand-to-hand/gun-to-gun combat. Their ranks consisted of Gun Clerics, Biotic Clerics, and Black Vanguard.
The Black Robes tradition
Their traditions are heavily based on the ancient Japanese samurai and the French Knights, and a sprinkle of the Christian church. There's Wikipedia for that, look 'em up.
This is in descending order from Major to Minor.
Gun Grand Cleric
Gun Master Cleric
Biotic Grand Cleric
Biotic Master Cleric
Their ranks are somewhat different.
Vanguard Grand Cleric
Used and practised with a passion by Gun Priests, Gun Kata is shooting the enemy as efficiently as possible while avoiding their fire. In summary, Gun Kata is the art of shooting where the enemy should be, while not being where the enemy should shoot. Through analysis of thousands of recorded gunfights, the Clerics have determined that the geometric distribution of antagonists in any gun battle is a statistically-predictable element. The Gun Kata treats the gun as a total weapon, each fluid position representing a maximum kill zone, inflicting maximum damage on the maximum number of opponents, while keeping the defender clear of the statistically-traditional trajectories of return fire. By the rote mastery of this art, your firing efficiency will rise by no less than 120%. The difference of a 63% increased lethal proficiency makes the master of the Gun Katas an adversary not to be taken lightly.
An exact copy of Gun Kata, with the weapon being Biotic Power instead of dual wielded weapons.
Black Vanguards use a mixture of both Gun Kata and Biotic Kata, and a little improvisation, normally wielding a handgun in one hand and having a free hand for biotic casting.