All effects are for 60 seconds on self:
This scroll's inscription reads:
Only two Scrolls of the Hidden Killer can be found. One is located on a table near Afer Flaccus in Raven Rock (cell -25,19). The other scroll can be found in the Tower in Raven Rock during the quest "The Assassin."
Additional scrolls can be obtained on the PC by using the console:
Player -> AddItem "sc_hiddenkiller" 1
A second (and identical) version of this scroll can be found in the game's Construction Set (item ID: sc_Chappy_sniper_test). Chappy refers to an NPC named "Chappy's Test guy" that also exists in the Construction Set but cannot be found in the game.