This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Lupin III: The Woman Called Fujiko Mine
The overall plot of Count Luis Yu Almeida. For no reasons beyond simply doing so, he began to experiment upon innocent girls with the most powerful psychosomatic hallucinogen on Earth. This was simply to ease the pain of them as they were robbed of their earthly possessions, electrocuted, raped, and poked and prodded on a slab. His overall plan was to create the perfect maiden which he could not do before his death. However, he did create Aisha, who proceeded to continue his work until her death. This is the absolute sadistic nature of Almeida.
How obsessed was Almeida with Fujiko? He built an entire theme park devoted to her enjoyment...or terror.
The mental scarring several characters suffer throughout the show lead to some terrible freak outs.
Several deaths throughout the show including Fujiko shooting two guards point blank and Oscar killing the Fujiko clones with a gasoline explosion.
Seriously anytime you start to see the Owlmen appear, expect this to not be far behind.