An Ancient Note is a written note found in Dishonored.
I am sorry, but I won't be here for the next meeting. Or ever again, for that matter. What happened here was unnatural and makes me sick to recall. I left the result of our labors here for you to keep. But I expect that the rats will consume it before you return. Goodbye.
When Farley Havelock asks Corvo Attano to investigate the sewers, the note can be found close to where the weepers and two runes are found. The note itself can be found there from when Corvo first arrives at the Hound Pits Pub, even though the rune it refers to does not exist until Corvo awakes on the day he is assigned the task.