The Eternal Flame a Dragon Soul

In the world, 90% of the population have special souls called Monster Souls. The origins of those special powers was a great war +4000 years ago. A group of demons chosen by The Demon King planned to take over the world, but they were stopped by The Seven Deadly Sins, a group of extremely powerful "humans" that had special souls and were raised by powerful monsters. The demons were all sealed in the safest place in the world and the mysterious humans disappeared. The story resumes with a family of 4 brothers: Aki who is 12, Haru and Rin who are twins and both 15, Wes who is 17, and finally Yukio who is 18. They meet a group of friends in each of their classes and some time later, Monsters and Demons start attacking Thrar and consequently Fostren, Britannia, Ebion, Refriaria, Skjanes, Fendou, Dakengtou, Atsuga, and finally Konoura. These can only be stopped by the legendary group that destroyed them +4000 years ago... but they're dead right?