Well, Assassin's Creed has really got into my head. Forget the gameplay and graphics and everything else, the story itself is a rumble. So, I've read the article on Assassins in wikipedia and am posting the gist of it here:-

It is commonly believed(but not confirmed) that the word "assassin" originated from the Persian word Hashashin/Hashishin/Hashashiyyin/Hashasheen- a designation for the Nizari branch of Ismaili Shiite Muslims. They parted ways with the Fatimid(bloodline of Fatima, Prophet Muhammad's daughter) Ismaili Empire and became an isolated political/theological group. They are popularly called Nizari and are currently led by Aga Khan IV, their 49th Imam(Imams are the "Pope" of Shiite Muslims).

The Fatimid Empire, split in the Empire and the birth of the Nizari group
Despite being a minority, within a minority, the Isma'ili under the leadership of their Imams succeeded in establishing a generational secretive underground movement against the Abbasid Caliphate. They based their ideas on Greek philosophy, and mysticism, and an end to perceived corruption and greed. They would turn their revolutionary ideals into reality by establishing the first Shia state; the Fatimid Empire, spanning across the Mediterranean and Levant. Its capital was Cairo in Egypt, culturally brilliant with some of the finest institutes of learning in the known world, the empire would bring scientific, and social breakthroughs to all its peoples, including religious freedom.

The eighth Fatimid Caliph and Isma'ili Imam Ma'ad al-Mustansir Billah took ill in Cairo, his powerful Vizier Al-Afdal took the reins of state power, following the death of the Caliph, Vizier Al-Afdal lead a palace coup d'etat, appointing his brother in-law the Caliphs younger son Ahmed whom he dubbed Al-Musta'ali. The heir apparent Nizār himself left for Alexandria where he was given strong local support and lead a rebellion, but he was eventually defeated and executed on his brothers orders. This caused a split in the Fatimid Empire amongst Isma'ili.

The group of Nizar's supporters, called the Nizari, continued under the strong and able leadership of Hasan-i-Sabbah(his rank and significance is something like that of "Al Mualim" in the game). Under his leadership, the Assassin's attacked anyone who posed a threat to the Nizari group. The group's empire was brought to an end by Moghul warlord Hulagu Khan with his attack on Alamut, the heart of the Nizari group. It was later resurrected by Aga Khan IV.

During the siege on Alamut, most of the libraries and written records about the Nizari group were destroyed. As a result, the only sources of information on the Nizari are the polemic of contemporary Arab historians and highly unreliable accounts of Marco Polo. Hence, fiction highly outnumbers the facts. A popular legend derives from Marco Polo, who claimed to have visited Alamut during his journey east, is that future assassins were subjected to rites similar to those of other mystery cults; the subject was made to believe that he was in imminent danger of death. The twist was that they were drugged to simulate "dying" and later they awakened in a garden flowing with wine and served a sumptuous feast by virgins. The supplicant was then convinced he was in Heaven and that the cult's leader, Hassan-i Sabbah, was a representative of the divinity and all his orders should be followed, even unto death. Much of the current western lore surrounding the Assassins roots from Marco Polo's supposed visit to Syrian fortress of Alamut in 1273 (a visit widely considered fictional since the stronghold had been destroyed by the Mongols in 1256), and from returning Crusaders from the Levant who encountered their local Syrian leader Rashid ad-Din Sinan (the old man of the mountain) in the fortress of Masyaf.

The Nizari group was so small that they couldn't even afford an army. So, they formed a covert group of sleeper commandos, who used a hit-and-run method for their missions. Due to the absence of an army, the group was inclined towards assassinations rather than warfare to eradicate political rivalries. These commandoes were called "Fedayeen"(or Assassins, if you may).

The Fedayeens were somewhat kind for murderers. They always gave their victims a deadline to reconsider their decisions and if the victims insisted on upsetting the Nizari, the Fedayeens moved in for the kill. For example-a victim, usually high-placed, might one morning find a Hashshashin dagger lying on his pillow upon awakening. This was a plain hint to the targeted individual that he was not safe anywhere, that maybe even his inner group of servants had been infiltrated by the assassins, and that whatever course of action had brought him into conflict with the Hashashashin would have to be stopped if he wanted to live. They were meticulous in killing the targeted individual, seeking to do so without any additional casualties and loss of innocent life, although they were careful to cultivate their terrifying reputation by slaying their victims in public. Typically, they approached using a disguise, or were already sleeper agents in an entourage. Preferring a small hidden blade or dagger, they rejected poison, bows and other weapons that allowed the attacker to escape and live. For unarmed combat, the Hashshashin practiced a fighting style called Janna which incorporated striking techniques, grappling and low kicks. However, under no circumstances did they commit suicide, preferring to be killed by their enemies once the assassination had taken place.

Notable victims of the Assassins-the Abbasid Vizier Nizam al-Mulk (1092), the Fatimad vizier al-Afdal (1122) (responsible for imprisoning Nizar), ibn al-Khashshab of Aleppo (1125), il-Bursuqi of Mosul (1126), Raymond II of Tripoli (1152), Conrad of Montferrat (1192), and Prince Edward (later Edward I of England) was wounded by a poisoned assassin dagger in 1271