War of Roaches [r/HFY]

Humanity, once a conqueror of the stars, is now pushed to the brink after thousands of years of war. Outclassed, outgunned, and increasingly outmanned, humanity places hopes for its survival on hidden seed worlds in the farthest reaches of the galaxy, but even those are hunted down, and many are simply lost to the noise of the cosmos.

One of those long-forgotten worlds, Earth, is targeted anew by the enemy. A fleet is scraped together to rescue Earth's billions, the sheer number maybe even enough to turn the tide of the war—but it is too late, and they are now only millions. The enemy proves too strong even for vengeance, and the fleet is reduced to stardust. Now stranded on a planet devastated by bio-weapons with no hope of rescue, the fleet's survivors must do what they can, not to achieve victory, but to make the opponent bleed.

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What to expect: Army-building, Nanite "magic system", Zombies, Uplifted survivors, Escalation to interplanetary war.

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Cover attribution: Image ("A group of armed forces walking in the desert. in the distance is a huge alien mothership floating in the air.") by liuzishan on Freepik.