League of Assassins (Arrowverse)

An ancient and mysterious society initially believed to be a mere myth, the aptly-named League of Assassins are a deadly organization and are led by Ra's al Ghul. They are the Bigger Bads of Season 1 in Arrow due to training Malcolm Merlyn, the season's main antagonist, and the quinary antagonists/anti-heroes of Season 2, this time done through Nyssa Al' Ghul leading a squadron of assassins, then the straight-forward, direct main antagonists of Season 3, as they now are at war with Malcolm and Oliver Queen/Green Arrow and seek to destroy Starling City.

Due to Malcolm learning his combat skills from them, Nyssa's hunt for Sara Lance and Ra's now trying to kill Oliver and Malcolm and destroy Starling City, one could argue that they are, in fact, the true main antagonists of Arrow.

Overview
Ra's al Ghul created the League of Assassins to hunt down and kill people he deems a danger to society and has done so for many years, resulting in thousands of people killed. All of its members go through a harsh and bitter training that leaves it's trainees either dead or at the peak of human physical condition, so much that they rarely feel pain itself. They are all trained in the arts of swordsmanship, archery, stealth and killing in both the quickest and most discreetest of methods or to make it painful and last for days. All of it's members also seem very devoted to Ra's as many of them have been seen killing themselves for the league, allowing Ra's to kill them simply for him to train himself and will carry out his orders without question.

The League also has a deep code of honor and those who violate the code are hunted down and killed by Ra's personally. The League also does not allow any of its members to leave unless they want to die and anyone who tries to run away from the league is hunted down and returned, alive or dead, or if they are unreachable and have families, the families are targeted instead. Despite this, Ra's took pity on Malcolm Merlyn and allowed him to leave with the understanding that Malcolm would obey their code, suggesting Ra's had a certain fondness for Malcolm. If a killer that draws the league's attention, doesn't claim responsibility and face justice the league targets 50 people a day until that person confesses and faces justice. However those who commit crimes against the league or break it's principles are allowed a trial by combat in a fight against Ra's himself, though most of them seem to be afraid to fight him and choose death upfront. When the trial does take place it is upon a mountain that can only be reached by a long and rigorous climb that some do not even survive. Furthermore all members of the league are given a new name of their choosing to be addressed by. Their only known place of establishment is Nanda Parbat.

The league tends to be very old fashioned in a lot of things, including communication methods, such as invisible ink that is made visible when exposed to a flame and signal messages, and their sense of justice, which includes the aforementioned killing of 50 people a day in retribution for a crime until the person who truly committed the crime was routed out. They also prefer bows and arrow, staffs, and other weapons from medieval times. They do, however use cellphones as a form of communication. They also train their members so they can escape from any prison.

The League's members take new names in the Arabic language; it appears that learning that language may be a requirement of membership, as many League members (even those whose native language is not Arabic) have been shown speaking it.

Each member has his/her signature weapon. During the course of their training members train to master all kinds of weaponry but eventually specialize in one weapon, and that will be the one they will carry with them at all times during their missions. Malcolm's weapon of choice was a bow, along with Nyssa who also carried a bow, Sara a staff, Al-Owal a sword, unknown members a tomahawk, and a wooden stick.

Members are also sent out with a poison that kills the user quickly and painlessly for members to use in the event they are captured to prevent interrogation. They are also very efficient in finding information, even something as faint as rumors of Malcolm Merlyn fathering an illegitimate child, something Malcolm himself wasn't aware of until recently, even though they weren't able to confirm the rumors. This child is Thea Queen, who is now being taught by Malcolm with the League's customs.

History
Prior to 1936, an 11 year old Ra's al Ghul killed a man who threatened his family's safety and though he felt sorrow for taking that man's life, he also realized he had given justice to the world by removing that man's evil with his death. From then on Ra's sought to fix the world by cleansing it of people who sought to do wrong also and created the League of Assassins to carry out this precise mission resulting in thousands of people being killed.

In later years, the League gave special training to Malcolm Merlyn and Sara Lance, and the both of them soon left the organization. Merlyn went on to dress up in League clothing with a bow and custom-made arrows to battle the Arrow, Oliver Queen as a vigilante. Sara betrayed the League and was pursued by Nyssa and other members of the League until Nyssa releases her from their control.

Hunting Sara Lance and Slade Wilson's attack
A unit from the League of Assassins led by Ra's daughter Nyssa went to Starling City to attack Sara Lance, since she had left the organization and their code says to kill her. She is pursued relentlessly until Nyssa decides to release Sara from the League's control. Nyssa later helps Oliver Queen, also known as the Arrow, battle against the minions of Slade Wilson as he tears Starling City apart. Nyssa kills Isabel Rochev during this, while the League leaves later after Slade's imprisonment on Lian Yu.

War with Oliver Queen and Malcolm Merlyn
Ra's later prioritized killing Oliver Queen and Malcolm Merlyn due to disrespecting the League's code of honor with Oliver refusing to kill his opponents despite Nyssa's insistence and Malcolm for creating the Undertaking conspiracy and demolishing the Glades, Starling City's lower-class district. Sara was eventually killed as Malcolm declares war on Ra's, hoping to overthrow him with help from his daughter Thea Queen (who is now being taught by Malcolm in the ways of the League, figuring that to kill an assassin and his code, send an assassin with a code). Oliver is soon challenged to battle Ra's atop a mountain, due to failing to find the killer of Sara Lance within the promised 48 hours. Oliver is even offered a chance to become the next leader of the League.

Known members

 * Ra's al Ghul (founder and leader)
 * Nyssa al Ghul (second-in-command; Ra's heir)
 * Maseo Yamashiro (known as Sarab; meaning Phantom)
 * Mesi Natifah (known as Talibah; meaning "Seeker of Knowledge", or "Student")

Former members

 * Malcolm Merlyn (known as Al Sa-Her; meaning The Magician)

Deceased members

 * Al-Owal (The First)
 * Sara Lance (known as Ta-er al-Sahfer; meaning The Canary)
 * Chase (League's spy)
 * Unnamed (League's sentry)

Season 2

 * "City of Heroes"  (Sara only)
 * "Broken Dolls"
 * "Crucible"  (Sara only)
 * "League of Assassins"
 * "Heir to the Demon"
 * "Unthinkable"

Season 3

 * "The Calm"  (Sara only)
 * "Corto Maltese"  (Nyssa only)
 * "The Magician"
 * "The Climb"
 * "Left Behind"  (Maseo only)
 * "Midnight City"
 * "Uprising"
 * "Canaries"  (Chase only)
 * "Nanda Parbat"

Trivia

 * This is the second live-action adaptation of The League of Assassins, though the first time by it's DC comic book name rather than the alternative name "League of Shadows".
 * In some incarnations, like Christopher Nolan's Batman trilogy, the League of Assassins is known as the League of Shadows. However it should be noted the original name is the League of Assassins as it was the name used in the DC comic books.