Read something here. I recommend The Seven Geases.
"May the ordure of demons bemire you from heel to crown!" cried the venomous ancient. "O lumbering, bawling idiot! you have ruined a most promising and important evocation. How you came here I cannot imagine. I have surrounded this place with twelve circles of illusion, whose effect is multiplied by their myriad intersections; and the chance that any intruder would ever find his way to my abode was mathematically small and insignificant. Ill was that chance which brought you here: for They that you have frightened away will not return until the high stars repeat a certain rare and quickly passing conjunction; and much wisdom is lost to me in the interim."