Hey, everybody, a "REAL" ad! My guess is that, in a comic mag like Skywald's NIGHTMARE, they had to make sure that the intended age group of its readers knew the difference between a comic story and advertising art, since they looked pretty much the same!
Yellow Phantom, over at the fabulous BLOOD-CURDLING BLOG OF MONSTER MASKS posted this very same ad a while back, only his was spotted in an issue of PSYCHO. Intended to compliment his previous post, this "Horror House" ad is from NIGHTMARE #1, from December 1970. I'd say that the art is by John Severin if I had to make a guess. "Gruesome to behold", indeed!