The world is under siege from interdimensional beings after and the only thing that stands in their way are a group of teenagers with attitude that grew up together in a remote japanese mountainside town famed for its colossal world tree. The tree itself had sentience and put up a barrier to defend itself but the five young girls and boys who happened to be camping there that day stood in terror as the grosteque behemoths broke in no time at all. Facing certain doom, those present prepared themselves to meet their maker but a demon saw it fit to intervene.
A voice calls out to them and presents a bargain - its protection and its blessings in exchange for their very existences. They only need to offer themselves up to it piece by piece and it shall serve as their guardian against these invaders. The eldest boy agrees and the demon quickly tells him to pull out the darkened sword at the roots of the world tree to summon it. Before the boy does so, another voice calls out for them to stop. This time it was a girl's voice emanating out from the tree and it pleaded them not to let the demon loose for they will suffer fates much worse than death. Naturally those words would not be enough to sway them as the boy pulls out the blade and summons the demon forth in all its mechanical glory.
The demon's retaliation against the invaders starts off well but they soon manage to push back against it. The demon demands the first offering of many and consumes the boy's right arm in darkness. New armor forms on the demon's right arm with the telltale glows and markings of power to it while the boy finds that though his arm remains wholly intact physically, he can no longer move it of his own will. Before the boy can truly process the horror inflicted upon him, the demon's dark essence surges into the lifeless arm and brings it back to life to serve its new master. The demon's voice now rings through his head and tells him to follow his lead and take a stance as they unleash its new powers on the remaining behemoths left for them to destroy.
In typical mecha motw fashion the story has the teens giving up more and more of themselves to the demon to protect their town as light is shed on the truths behind the tree, the demon and the invaders. It is only when one of the girls offers herself up fully that they finally learn that the demon and the tree were exiled lovers who committed a forbidden taboo eons ago and the invaders came because of the crime they committed. Furthermore, the demon never had the means to permanently keep the invaders at bay and that his goal to acquire two of the teens' bodies for themselves and enough existence energy to make their own escape. It comes down to the MC to decide if he's willing to sacrifice one of the other two girls in exchange for the one he loves or if he'll do the pragmatic thing and offer himself up completely to let the two escape and lead the invaders away from them.