Tense, low-key ambient that knows exactly what it's doing for an X-COM spiritual successor. UFO: Alien Invasion is an open-source tactical game, and its seven-track soundtrack keeps things moody and utilitarian. The Geoscape Music has that slow, building dread you need while staring at a world map waiting for the next alien contact.
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The Mission tracks layer in subtle percussion and uneasy synth pads that stay out of your way while you're calculating shot percentages. Minimal but effective. The Main Theme ties it together with a militaristic edge that doesn't overstay its welcome.
If you've burned through every X-COM game and need another turn-based alien hunt with the right atmosphere, this one's free and sounds the part.