Many Game Masters dislike the way Cthulhu Mythos Magic is handled in GURPS Cthulhupunk . Primarly due to the requirement of having to roll a "Cthulhu Mythos Fright Check" every time a mage casts a spell. This creates an additional roll that can slow down game play, especially during combat. However, GURPS Fantasy 4th Edition , seems to have the makings of a good alternative.
Every time someone casts a Cthulhu Mythos spell, it would not only cost fatigue, but the expended Fatigue Points would also contribute to a separate "insanity" total. This total, tracked by the GM - not the player, would accumulate with each cast spell, until it reached a predetermined threshold which, when reached, resets to zero, and the character receives a 5 point disadvantage or an existing disadvantage worsens by 5 more points. The GM could choose a threshold, for instance 25, 50, or even a variable number which is rerolled after each reset, such as 2dx5, depending on the desired austerity of the campaign. The GM may chooses the disadvantage gained, and may choose to limit them to mental disadvantages. Although many gruesome physical, some social, and certainly supernatural disadvantages also seem appropriate.
If a less austerior game is desired the "insanity total" might decrease naturally over time. For instance, 1 point per game week, 1 point per game month or 1d for each full game week without casting a spell. This allows the character to use magic sparingly and for emergencies without quickly degenerating into a unplayable monster.
The GM shouldn't tell the players how fast (if at all) their total is reduced, or the threshold necessary to receive the resulting character point penalty. Instead, the GM should find a way to inform the player, at an appropriate time, about the new problem his character has developed.
Another alternative would be to allow for certain mystics and mages to have developed esoteric skills that reduce "insanity" point total at a faster rate. For example, an additional point per day of meditation with a successful skill roll. However, in most cosmic horror style campaigns, it is probably advisable to make such skills difficult to obtain.
edited: 12/01/07 by Judd Fetters
note: the above was edited to correct spelling errors, to improve readability, and to conform to the general SJ Games style guide (http://www.sjgames.com/general/guidelines/authors/style.html). All effort was made to ensure content was not altered