[Warning, pretty big Undertale spoilers below. Stop reading if you want to avoid spoilers].
The first time I beat Asgore on Undertale’s pacifist route, only to have Flowey come in and force the game to close, I sat in shock. A mechanic had been used; a mechanic the game had never hinted at before. I knew Flowey was likely to end up being a boss I had to fight, and I knew he could remember things in the game even if I did not save them, but the fact he might be able to manually close the game without my control was new and scary to face up against.
However, Flowey being able to control closing my game window was far from the scariest thing about fighting him. Flowey as a character breaks all the rules of how Undertale, up until that point, had operated.
Undertale was developed under arbitrary constraints. Enemies were designed around simple line art, with minimal detail and very little in the way of character animation. Everything was deliberately retro in design, and that was all the game had ever suggested it was going to be.
Flowey turned up to fight, with a face fluidly moving between expressions and laughter properly synced up to his mouth movements. As his body came into view, he appeared to have been designed in full colour. His movements were flexible, unpredictable, and not on set predictable paths.
His attacks were photographic. Explosions taken from reality. All the visual rules of the game had been broken, and I was intimidated.
For my money, Flowey is one of the scariest villains to fight in any video game and that’s mainly thanks to him pulling the rug out from under the game’s metaphorical feet.
What better way to encourage fear than to present your villain as a complete order of magnitude above expectations set up by the game.
Holding back Flowey’s visual style until the last moment, using it as a tool to represent the immense threat his newfound power possessed, was a move of genius that made him stick in player minds far better than perhaps any other villain.
Sometimes it pays off to develop most of your game under artificial constraints. Getting to make an immense leap in presentation when your villain jumps in power is hard to outdo.