Story by John Tynes. First published in a limited edition chapbook by Armitage House, it has been reprinted several times, most recently in Rehearsals for Oblivion (Act I). It forms a loose trilogy with Broadalbin and Sosostris.
It is a tale of the Carcosa Mythos; it reportedly grew from a bedtime story that Tynes told his daughter, about an old man living all alone in the vast, empty city of Carcosa. The Ambrose of the title is supposed to be Ambrose Bierce, the great American writer whose story An Inhabitant of Carcosa laid the groundwork for the entire Carcosa mythos, and who vanished without a trace in 1914. In keeping with Bierce's story, it presents Carcosa as a place of loneliness rather than horror.
The story itself can be found online HERE.