Fallen Letter is a written note in The Knife of Dunwall DLC.
Transcript
YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER YOU KILLED HER
Location
It can be found during Daud's visit to The Void, next to the body of the slain Empress in The Knife of Dunwall.