Stargate SG-1/Nightmare Fuel

• Anubis, both before and after "Lost City". Also the Ori and possessed Doci.
• The Goa'uld themselves. These foot-long snake-worm-things with fins will take complete control of your body and use it to live like a Complete Monster for thousands of years. They have access to all the original person's memories, so if they want, they can impersonate that victim almost seamlessly.
• The Replicator-ized Merrick from The Ark of Truth certainly qualifies.
• The Replicators in general qualify. Nigh indestructible techno-bugs that devour anything in their path and get smarter the more there are of them? Not to mention the Mind Rape the human-form Replicators seem to enjoy so much. Yikes.
• See Cruel and Unusual Death. Even if it doesn't happen onscreen it is highly disturbing.
• The Ori. If the concept of being burned alive for not worshiping isn't nightmarish enough, that fire skull form is.
• Season 9 Episode 17... "Scourge." Prior bugs that bite you, lay their eggs in you and then eat you from inside out. If you're lucky they'll just consume your entire body in seconds.