[cromulen]

Alamut by Vladimir Bartol

Written in 1030's by a Slovenian, it's a historical fiction novel set in the in 11th century what is now Iran. It tells the story of how a radical leader of an Islamic faction, based in the impenetrable mountain fortress "Alamut", manipulates young men into perfect obedience and turns them against his enemies through the use of drugs and a fake paradise full of women purporting to be heaven.

It is also one of the major inspirations for the Assassin's Creed series of games.

[Pamar]

I believe it inspired much more than the videogames: https://en.m.wikipedia.org/wiki/Assassins