Le Roi en Jeune
From Tatters of the King
The Bibliotheque nationale has a single copy of this privately published play. Talbot Estus claims to own a copy as well, and to have based his play, 'Carcosa' on it.
Though not the notorious work of Chambers' stories this play was indeed suppressed and burned by the French authorities, for reasons that are unclear, and there are rumours of madness and death surrounding it.
The play predates Chambers' work so seems to have been an inspiration for him as well.
There is evidence of older works with the same name - a fragment by Marlowe and a lost Restoration drama, but neither is available for perusal.