Hy there,
some time ago I had an encounter in R'lyeh and drew the red/yellow-dual-color-card and had to fight Cthulhu on my own. It was a close call, but I finally warded him off (my equipment was quite nice as I was playing Joe Diamond and wielded dual .45 automatic handguns). So I was asking myself, does his attack lose it's effect after the battle and my health and sanity-maximums are returned to normal or is the effect permanent?