One of the few published Phantasm Adventure modules. The only other official module was called the Maldake Adventure. The Black Keep of Serpent Lake was published in 1988 by Dai Nippon Kaiga, the Japanese publisher of the game.
I cannot remember much about the game, and sadly the English version has been lost in time. It do remember writing a number of module packs for the game, but do not remember having a lot of contact with Dai Nippon Kaiga after the game was published.
I am still in negoitations with the Dai Nippon Kaiga and another Japanese game company about re-printing the game. If you would like to see another print, I ask you to write Dai Nippon Kaiga and express your interest: firstname.lastname@example.org