3Omega Flowey (Undertale)
Flowey in Undertale was a bit of a rogue element. While he is pretty scary, we didn’t expect him to truly terrify us. And then he turns into Omega Flowey. After absorbing human souls, Flowey transforms into a huge, grotesque creature with a giant mouth and weird tentacles and all manner of other disturbing accoutrements.
The worst part of the battle, and the part that scared us the most, is when Flowey seems to be almost defeated and then simply “reloads a new save file” making it so he regains all of his health. The dread the player feels in that moment is real and still terrifies us to this day. Undertale might not be a horror game, but Omega Flowey is one of the scariest bosses we could think of.