Based on Hashshashin Armor by Dimitri Mazieres:
http://www.nexusmods.com/oblivion/mods/8043/?

This small plugin aims to incorporate the Hashshashin Armor into The Elder Scrolls lore, through a book that gives the Hashshashins a backstory while avoiding anything too specific. This way no matter who your character is, you can enjoy the armor without its lore conflicting with your own story.

Hashshashins were actually a real-life group of assassins in the middle ages, which Assassin's Creed is inspired and loosely based on. When I thought the lore I aimed to incorporate characteristics from both the actual Hashshashins and the game and combine it with TES history.

You DO NOT need Hashshashin Armor for this. It's a simple standalone book that simply gives it backstory.

The book is basically a brief documentary explaining who the Hashshashins are. Because it's not commonly produced, it's worth 100 gold and raises your Marskman skill.

The book is found on the top floor of the Abandoned House in Cheydinhal, on the shelf. Over time I do plan to expand this by going in further depth, mixing more nonfictional facts in, further incorporating further TES lore into it and giving the book more locations to be found.