Every five hundred years, the gates of heaven will open, allowing the Angels to commence their invasion. Based on fundamentals, they don't attack human settlements all that often, but overly developed towns end up destroyed.
Research indicates that the Angels despise civilization as the yang to their yin if you will. The only ones that have managed to escape from their onslaught is Dwargon. Their domain only has two entrances. Large scale invasions don't work either because dragons inhabit the mountain ranges. Therefore, they've managed to defend themselves somehow. That's more or less the reason why civilization is stagnated to a certain level. The only exception is the Ingracia Kingdom. It's also the reason why the council is there in the first place. It's in order to perform research and try to mitigate the damage of each invasion.
Every five hundred years, the Imperial Capital is redesigned based on how much damage is caused after an invasion. It's basically trial and error. This is something that each country has unanimously agreed on and something that all residents understand completely. It's also because their research hasn't been over the top that the destruction and devastation has been kept to a minimum.
Each nation gives out support and carries out its research in total secret. Any discoveries are generally concealed and only the kings of each country can utilize the latest in technological development. If they were to spread the results to the people, the angels would certainly unleash their full wrath upon them. This system was carefully determined and agreed upon. Only discoveries that are determined to not cause incident are released to the masses.
The purpose of the angels are to eliminate monsters as well.
To put them under control The Ultimate Skill Justice King Michael is used under which an ability Armageddon is initiated which Summons and Controls an Army of 1 Million Angels. The users of this skill are Star King Dragon Veldanava (Original User) Rudra Nam Ul Nasca (Borrower) Yuuki Kagurazaka (Stealer)
- Seraphim: There were formerly 13 members of the Seraphim. But three of them had fallen, including Dino.
Angels are individually close to a B+ rank, so they always come in armies of roughly 1,000,000 strong or more. In addition, there are leaders of captain/commander classes that are stronger and organize the invasion. There are general level classes as well, but the combat capabilities of these individuals are unknown.
They're also under the Holy attribute, so any real attacks or intervention from the church will probably be useless. However, they don't seem to be allies of humanity either.
- It is hinted in "Rimuru's Elegant Escape - 09" that, in fact, Rudra Nam Ul Nasca was responsible for the release of the Angles and their attacks on civilization, as an attempt to make humankind cease war with each other.