Fear bombs are a Jack the Ripper expansion tool: hallucinogenic grenades that scream on impact and send most enemies running in terror. Evie learned them from the Indian Brotherhood, and they let you clear a group without killing anyone.
The blast lets out a demonic scream and a cloud of hallucinogenic powder. Enemies in the radius see their worst fears and flee, which suits a non-lethal approach even though the bombs are anything but quiet.
The Indian Brotherhood used fear tactics to reduce bloodshed, and Jacob and his trainees picked them up during a visit to India around 1873. By 1916 the British Assassins no longer use them.
Jack the Ripper also carries fear bombs, but he uses them to torment his victims rather than spare them.