"Call of Cthulhu alternative rule system" From the introduction: This system has been devised to allow a fast and renewed use of previously published Call of Cthulhu D100 supplements. Despite being completely different from the original D100 system Mad Cthulhu’s mono D10 system can be used to play CoC adventures without the need of a massive conversion. A Game Master can just quickly convert the adventure every time he runs into monsters, NPCs, weapons, skill test and so on.