Complex sentence: To his horror they were all victims of a sexual sadist serial killer who operated out of a farm house converted into a murder hotel, and more specifically based on the one used by real life American killer H.H. Holmes.
Split into simpler sentences: To his horror they were all victims of a sexual sadist serial killer who operated out of a farm house which had been converted into a murder hotel. More specifically, the hotel had been based upon the one used by real life American killer H.H. Holmes.