MC is an aimless NEET that gets reincarnated in a technomagic world ala truck-kun with his core memories. He is reborn as a little girl, one of many that were grown out of tubes to become future technomagic hybrid machine soldiers created to reclaim a city lost to self aware machines.
Contrary to her expectations from reading light novels in her previous life, this life proves to be a difficult one from the outset as she is treated as an expendable and abusable object. Resources and populations continue to dwindle as humanity fights a seemingly endless war of attrition but MC survives enough hardship to become an elite fighter capable of fulfilling her ultimate role.
After an extremely bitter battle that cost her many of her comrades, she makes her way to the city's high command only to discover a lone immortal magisoldier like herself running it. They battle and MC deals her a fatal blow but the immortal reveals that the story of machines taking over was made up to give humans a cause for war.
For centuries, a mysterious phenomenon has plagued their world where if a certain degree of harmony is achieved within a significant enough population, humans will begin forming hive minds that give rise to terrible monsters that go on to consume other human settlements and grow even more monstrous. There hasn't been an outbreak in as long as she's been in control but having grown so weary over time she gladly welcomes her death.
With the fate of their world now in her hands, MC now has to make some very difficult choices as the mantle is left to her.