Robert W Chambers invented this fictitious play, which has
the effect of driving its readers picturesquely insane. He
only gives a few lines of it in his stories that mention it,
which you may read here: www.yankeeclassic.com 
but that hasn't kept three or four people from trying to imagine
the rest:
Thomas Ryng (available from Armitage House):
www.tccorp.com
James Blish (in the short story, "More Light")
Valerian Caprice (Act I only)
& an appropriately anonymous version
The King In Yellow By Graywyvern. Anonymous? The identity of Graywyvern can be discovered by googling him and looking at the eleventh hit, which is on result page two. (Sherlock will be proud...)
See: myweb.worldnet.net (Broken Link in 2002, archived at web.archive.org ) for a very comprehensive overview.
For an idea of what it has spawned, see: www.yellowsign.homestead.com (Broken Link in 2002)
For a movie based on Chambers's book see: www.carollus.demon.co.uk (Broken Link, archived at web.archive.org , but see also www.repairerofreputations.com and www.unfilmable.com )
See original on c2.com