Assassins in Dungeons & Dragons are shadowy figures who can kill with a single strike. Assassins appeared in Men & Magic as a special class (Arneson 1975:1). They were masters of disguise and poison, paid by performing assassination missions and given titles that matched more of the Indian style of Thuggee (the sixth level title is Dacoit, the seventh Thug). As evil characters, it was difficult to imagine that these characters could ever be a fully-functioning member of an adventuring party.
The term “assassin” was coined in 1531 via French and Italian (Ayto 1990:39). The belief that assassins imbibed “hashish” helped form the word “hashishiyyin,” which means “hashish-users.” Active in Persia and Syria from the 8th through the 14th centuries, the original assassins were members of the Nizaris, a fanatical Ismali Muslim sect who opposed the Abbasid caliphate through murder. The leader of the assassins was the Shaik-al-Jibal (“Old Man of the Mountains). Although the two classes have their roots in different cultures, the assassin (Ismali) and the paladin (Knights Templar) actually met during the Crusades.
Much of what was known about assassins in the Western world was drawn from Marco Polo, who explained how the assassins were drugged with special potions that made them both fearless and hopeful for an afterlife filled with pleasure. Polo’s account is undoubtedly biased, but it is his description that made its way into fantasy culture.
The assassin was removed from 2nd Edition, introduced as a prestige class in 3rd Edition, and restored as an exclusive class in D&D Inside for 4th Edition.
This description is a rough draft from my upcoming book, The Evolution of Fantasy Role-Playing Games.