I'm looking for a manga that consists of the protagonist going into a similar world to his, where the mystical beings that were put in prisons are free and there is war. He meets a chained angel girl and sets her free but there are monsters that want to kill her. There is also a futuristic sword in the same place.
If you could find the name, it would be a huge help because I can't find it.
Perhaps you're thinking of Why Doesn't Anybody Remember My World?.
From Baka-Updates:
"Why does no one remember the true world?!"
The era when the great race of the five tribes competing for hegemony on the ground ended with the victory of mankind led by the hero Sid. However, that world was suddenly "overwritten" in front of a boy named Kai. In the rewritten world, Kai saw the scene where humans lost the five tribal wars because of the absence of hero Sid. Here dragons and demons dominate the ground, and Kai is a forgotten existence from all human beings. However, after encountering the mysterious girl Rinne, Kai decides to break this rewritten destiny. In a world without heroes, he inherits the hero's sword and martial art to challenge the mighty enemy races who reign supreme.
A century ago, there was a war between the five 'great races' of humans, demons, angels, divine barbarics (demi-humans akin to elves), and mythical beasts. Humans were initially losing the war, but a legendary, sword-wielding hero named Sid helped them to turn the tide and imprison the other four races within black pyramids referred to as graveyards.
In the present, humanity is now the dominant lifeform, with modern cities and somewhat futuristic technology. The protagonist, Kai, is a young soldier employed by the Humanity Protection Agency, an organisation established as a safeguard in case the other four races were ever to break free from their imprisonment. He works alongside other young soldiers, and carries a weapon that's a hybrid of a gun and a sword. He also recalls having seen Sid's legendary sword within the demons' graveyard ten years earlier, although it's unclear if he was literally there or if this was some type of vision.
One day, Kai is walking through the city with a friend, when he notices a storm of some sort erupting all around, with the sky turning dark and the pristine buildings transforming into dilapidated ruins. He's the only one that seems to notice this, though, and after the storm has passed, he's left alone in this transformed, post-apocalyptic landscape.
As the story progresses, it transpires that the world Kai knew has been overwritten, and he's now in a new timeline where the hero, Sid, never appeared, and humanity lost the war with the other races. The other races now dominate the surface, humans live in an underground city, and no one remembers Kai or the Humanity Protection Agency, not even the people who were his comrades in the previous timeline.
Kai discovers that the demons' graveyard still exists in this timeline, although he's the only human who knows what it is. He ventures into it, hoping to find Sid's sword, which he does, along with a winged girl chained to a pillar. Her name is Rinne, and she was imprisoned there by a demon named Vanessa. Kai frees Rinne from the chains, and shortly after that, they're both attacked by Vanessa.
