An Assassin (刺客, shikaku) is someone who attacks and/or murders someone else by a sudden and/or secret attack, usually for political reasons.
In the Extra stage in Imperishable Night, Kaguya Houraisan sent the heroines as assassins in order to fight Fujiwara no Mokou. In Hisoutensoku, the Unnamed Giant Catfish had also sent assassins after Meiling. Also, in Cage in Lunatic Runagate, Reisen Udongein Inaba was worried about assassins coming from the Moon. However, it is important to note that there are no known assassins in Touhou Project.
In the Seihou Project's first work Shuusou Gyoku, Erich hired some people to assassinate VIVIT. This wasn't to actually kill her, but to test her ability to see if she can rescue Vivit, who is Erich's daughter. The certain characters were describe to be assassins in the ending of that game.
- Wikipedia entry for Assassin