Welcome to Union (Male Reader X Crossover Harem)

Across the universe there are worlds that possess powers, some are hidden, others rely on them, some are use in machines and others are use for fun. However they all share common interests by protecting, defending and fighting off anything wether it would be monsters or villains in all shapes and sizes. The problem was that they all are on separate worlds, no way of traveling to these worlds or even know if they're truly are other living beings. Luckily there is an answer for that, what if there is a school where they are united. Let's travel with these alternate worlds where they all unite together and share their experiences together in a newly built school