Pure dread in audio form. SCP: Secret Laboratory nails that feeling of walking through fluorescent-lit hallways knowing something is very wrong. Light Containment Zone Ambient is barely music, just this low hum and distant metallic scraping that gets under your skin.
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Then The Final Flash of Existence hits with this massive orchestral swell that feels way too cinematic for a free multiplayer game. They did multiple versions of it and every one goes off. Melancholy lives up to its name with slow, heavy piano that sticks with you.
Even the Famicom-style menu theme has charm. If you've ever been chased through Site-02 at 1 AM screaming, this soundtrack will give you flashbacks in the best way.