Founded by Hasan ibn Al-Sabah, aka Sheik-al-Jebal (the Old Man in the Mountain) in 1090, the Hashshashin were a brotherhood of Ismaili Muslims who wished to establish an Ismaili state in the Middle East, and used tactics from assassination to all-out warfare to that end. They even aligned themselves with the Knights Templar from time to time. Much like their Christian allies, the Hashshashin made many enemies, and in 1256, the Mongol Empire besieged the society's headquarters in Alamut, and using Chinese-wrought siege engines, managed to destroy the mountaintop fortress.
And while that's where the history of the historical Hashshashin ends, the society didn't completely disappear. Instead, the vanished into the shadows, hiding in the underworld of cities and kingdoms from Asia Minor to the Himalayas. The Hashshashin managed to preserve a number of relics and writings from the Sacking of Baghdad--more than even the Orders of Atlantis or the Veil Treaty--and they took it upon themselves to preserve the legacy of the Golden Age of Islamic Magic. Over the centuries, they abandoned much of their original ideology and instead came to see themselves as warriors against corruption and decadence, those who fought against decline to return their societies to an age of glory. If that required assassination, so be it. If that required saving a life, so be it. It was all the bigger picture.
Keeping to the shadows, they spread their influence through the Middle East and South and Central Asia, acting as neutral agents and trusted middle men for the various governments of the regions. They would sometimes work as hired blades, but as often they would as viziers or messengers or even guardians. They also managed to become heavily involved in both the drug trade and human trafficking--as the Hashshashin would justify, sometimes injustice for a few can lead to justice for more. At least that's what they tell themselves.
But make no mistake, the Hashshashin are skill the best assassins in the world. They have honed their signature craft into an art form. Combining magic and intensive martial training, they are almost unparalleled warriors and spies, able to blend flawlessly with crowds, and slip away from the scene of a kill. Many Hashshashin are also capable diplomats and rabble-rousers, skilled in fermenting uprisings and encouraging chaos among the masses. It was a Hashshashin agent who encouraged the formation the Black Hand which ultimately killed Franz Ferdinand, after all.
The blade of the Hashshashin has been alleged in countless other high-profile assassinations over the centuries, over the past few decades it's claimed they contributed to the deaths of Indira Gandhi, JFK, Alexander Litvinenko, Malcolm X, and countless others. Of course, this is only touching on the surface the Hashshashin's actions. They've become a major player in the supernatural world by literally killing their way into power. Dragons, demons, elder fey, archmages... it doesn't matter. As the Hashshashin say: death comes for all.