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Nightmare Fuel: The Force Unleashed
Facing Starkiller's Evil Doppelganger, who lets out quite a few animalistic screeches and yells throughout the whole fight, in the Jedi Temple. And the aftermath where Starkiller briefly hallucinates that he's become him.
Dark Apprentice: You will never escape me.
In the Dark Side Ending, Emperor Palpatine turning Starkiller into a cyborg, as Starkiller screams in terror and agony.
Starkiller explores the Dark Side Cave of Dagobah, where he encounters many other clones that are entangled by vines and eerily calling out for help.
The first time you're on the Salvation; it's dark, you're mostly alone, and your comlink occasionally broadcasts Rebel soldiers as they're being butchered by unseen forces. They don't call them 'Terror Troopers' for nothing. The fact that you're up against the most ruthless bounty hunter in the galaxy doesn't help even if you don't end up fighting him at all.
On Kamino, as Starkiller walks through a hallway, he's occasionally haunted by Juno and Kota's voices berating him. There's also a few surprise attacks from an illusionary Darth Vader and the cloning tanks you find along the way. The mist, creepy music, and haunting atmosphere make it all the more chilling.
In the Dark Side Ending, Vader's true apprentice just looks at Juno's dead body with absolute contempt before moving on, even though he's also a clone of Starkiller. He was hiding there the whole time and was able to take out both Starkiller and Kota almost effortlessly.