This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Great Mazinger
Hardias, General of the Evil Spirits Host (one of the seven squads of the Mykene Army) is a giant skeleton, wearing tattered rags, with his skull turn upside down, holding scythe with his right hand as his left hand has been replaced by a bearded head. It is downright creepy.
The scenes where Psycho Veia was putting a curse on Tetsuya to kill him slowly. Tetsuya was twisting in pain, clutching his chest as the Evil Spirit Warbeast was completely safe as she killed him slowly, stabbing a giant Great Mazinger doll into a cave barely lit up by candles emitted a ghastly greenish light as a creepy music echoed in the background.
Marquis Yanus has two faces: a human-looking one she uses to blend among humans and her true face, located on the "back" of her head. When she wants to switch to her true face, her neck turns one-hundred-degrees and her locks part sideways, revealing a demoniac face underneath it (She seems a demon or a Hanya). It is so nightmarish like it sounds.