This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Little Mermaid II: Return to the Sea
Morgana trying to feed baby!Melody to her huge shark minion.
Morgana's ultimate fate of being completely imobilized in a chunk of ice may have some people thinking she got off way easier than her big sister. Until you look more closely at her face as shes sinking, her eyes were still moving!
How about when Melody almost drowned? She even beat on the ice wall with her bare hands and earlier said it was too thick to try and break through. Seeing a 12-year-old girl trying to save herself from drowning by frantically beating on an ice wall....it's terrifying.
And to think that Morgana knew that this would happen to Melody.