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Nightmare Fuel: Super Mario Sunshine
The blackened Shine Tower.
The gaint eel in Noki Bay. He's huge and you have to fight him in a dark, underwater trench. Oh and he attacks by trying to eat you! Especially bad if you're not good with water.
Phantamanta. It's a large, paper-thin orange shadow in the shape of a manta. Wherever it goes, it leaves a trail of goop and emits a really creepy shriek when you try to spray it. The worst part? It's an Asteroids Monster and divides into smaller faster shadows as it is destroyed. When you've finally destroyed half of the original manta, the remaining mantas stop fooling around and all converge on your position at once with incredible speed at a time when you're probably almost out of health.
You may think that going in the lake would protect you from it, but it just hovers over any water! Nowhere is safe!