Zerg Invasion: I Have a Doomsday Fortress

In AD 2193, the Omega protozoa invaded Terra. The prosperous human civilization was destroyed in one fell swoop, and was replaced by a near-eternal apocalypse with no end in sight. The high-rise buildings of the past have turned into a breeding ground for insect swarms, and the fertile land has turned into a scorched earth spewing poisonous miasma... Order collapsed and the evil side of human nature took over. The surviving humans could only tear off the fig leaf of civilization and return to barbarism, relying on the creations left over from the golden age to survive in the ruins of the city. But the demise of mankind is not a destined outcome. Just as Terra was falling into the abyss to be completely swallowed by the swarm, an advanced civilization called the Observer noticed the planet that was about to be destroyed and decided to give it a little help... From alien legions to giant battle bodies, from the core technology of mechanical ascension to the enlightenment of psychic technology... The assistance provided by the Observers is enough to help Terra deal with any crisis that may arise. But there is never a free lunch in the world. As a condition for providing assistance, the Observer selected an ordinary human living on Terra and made a small deal with him...