Dangerous First-Class Monsters (Japanese 第一級の危険モンスター) are said to be monsters with an unnatural presence about them. They are known to share the title of Black Dragon (Japanese 黒龍) or have some relation to a Black Dragon.
What Is Known About Them?
Very little is known about Dangerous First-Class Monsters to a point where there is even no documents or other forms of material to go by about them. All of them are hidden away as Monsters of Legend, ??????, Unknown, or Unidentified. Many things about them, such as where they live and their abilities, is nearly assumption. It is said by the Hunter's Guild that these monsters can bring the world to destruction if they wanted to.
In some cases, the Hunter's Guild will secretly send elite hunters to search for these monsters in wherever they supposedly live. Generally, most elite hunters never return but a very small number do.