This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Lusternia
All of Magnagora, Shallamar, and the Earth Plane were subject to the Taint (transforming legions of animals and people into undead and monsters), but only King Gorgaliel was directly linked to the Stone of Truth as it channeled raw Taint through it. The result? — the transformation of noble, eloquent Physical God Gorgaliel into Gorgulu the Devourer of Souls, patron saint of Body Horror. A massive, amorphous blob of flesh, he's covered in mouths and eyes of various sizes, and exits solely to eat, sometimes devouring his own followers. Most disturbingly, he often manifests his pliable flesh into various semi humanoid monstrosities, connected to him via fragile umbilical cords of sinew. He sometimes eats these "babies" too.
The other Demon Lords are no slouches when it comes to terrifying, either. Nifilhema has peeled back her own flesh, and holds the strips of meat in place with silver hooks so her musculature is always on show; and Ashtorath sometimes enters rage states so fierce that his jaw locks up and he can't even scream, and emits such heat that the stone floor of his tower buckles and melts beneath him.
Some of the random dreams that can occur when you fall asleep. They range from the odd (dreaming that you are an eagle, swooping to catch a poor rabbit), to the appropriately macabre (Magnagorans can dream about holding a Masquerade Ball dinner party while the screams of tortured merians echo in the background), to the downright creepy (dreaming that you at home, laughing with your family and friends, when the walls cave in to reveal the empty void of space, and the appendages of some unseen horror drag you and your loved ones screaming into the nothingness).
The Astral Plane. It consists of twelve spheres floating in an empty void, bumping into each other randomly, each populated by a particular breed of hellish monster, ranging from steam breathing crimson-eyed steel goats to emaciated, bleeding eyed women in robes.
Once you really delve into the deeper and more disturbing aspects of Hallifax and Gaudiguch, like, -really- delve into it, you will never look upon the Crystal City or the City of Freedom the same way again. Magnagora got nothing on them if you look the right place.