Book in the universe of Tekumel
containing detailed descriptions of its Gods
for use by "dark" sorcerers, with demon strengths, spells
for summoning, etc. clearly spelled out (pun intended) for use in play.
Athem. If that were all, it would be as dull as most RPG materials, except that it was singled out by our friends, the Christian Right as an example of real spells in an RPG. (OOOhhhh, spooky!!) Better than that, they tended to call for sacrifices...of a particularly gory nature...(Oh the horror! the horror!)..and thus was cited as a possible goad to ritual sacrifice in the "real" world.
Unfortunately, their bubble was burst when it was pointed out that the author had purposely made the sacrifices so extreme that it would be impossible for anyone (short of a despotic Asian dictator) to carry them out -- one particularly powerful demon requires a sacrifice of five thousand slaves, each one enucleated and the eye sockets filled with rubies.
I dare any disaffected loner to try to carry that one out.