Carcosa is a game setting for the 1974 Dungeons and Dragons and 2011 Lamentations of the Flame Princess roleplaying games. It was originally a homemade product billed as 'Supplement V' for Dungeons and Dragons (with rules modifications) but has been recently been republished at a greatly-expanded page count for the Lamentations of the Flame Princess ruleset (including the previously released encounter PDF and adventure released for the original).
The setting provides a Cthulhu Mythos setting for 'swords and sorcery' adventure in which characters can battle the Great Old Ones with sorcerous rituals, psionics and strange technologies, although it does not appear to have too much to do, specifically, with The Carcosa Mythos proper. The original release gained some notoriety for dubious content, although this was greatly exaggerated and an expurgated edition existed with the same page count as the original due to the minimal editing required to remove 'offensive' content. There is no expurgated version of the 2011 release.