Ashbless was invented by Powers and Blaylock when they were in school; they quoted extracts from Ashbless' poems in their English essays, and reportedly fooled their teacher into believing him to be real.
After leaving school, Powers and Blaylock both became novelists, and each started referring to and "quoting" Ashbless in his novels. Initially, each author was doing this independently, but they quickly realised what had happened and started consulting one another so that their references would be consistent.
William Ashbless is also a major character in Powers's novel The Anubis Gates, the protagonist of which is Ashbless's biographer. It was this book that first mentioned "The Twelve Hours of the Night", said to be Ashbless's most famous poem.
Powers and Blaylock refer to each other in their novels; there is a "Captain Powers" in one of Blaylock's books (Homunculus), and in one of Powers' there is a ship named the "Blaylock".
In 1985, Powers and Blaylock produced Offering the Bicentennial Edition of the Complete Twelve Hours of the Night: 1785-1985, a prospectus for a non-existent collection of Ashbless poetry. The prospectus included a sample poem and a replica of Ashbless's signature (the "William" was signed by one, and the "Ashbless" by the other, of the authors).
This was followed, in 2001, by On Pirates (ISBN 1931081220) - the introduction was by Powers, the illustrations by Blaylock, and the text supposedly by Ashbless - and, in 2002, by The William Ashbless Memorial Cookbook.