[OP: ] (Unfinished! Please wait!)
[ "It is very important that you defeat this being. Treat it as the final battle, the last fight you must win in this Dungeon." ]
There was a need for a start, a body, some breaking points, and a certain end for a story to complete.
It was only a story when the start was certain and the end was not. One who knew about them, a conscious being that existed in this world existing on the base of the stories knew instinctively about their existence.
[The creature they were facing was a master that had repeatedly refined its skills in a confined space, dealt with the existing challenges of this space, and increased its strength through pure knowledge and self improvement.]
First of all, it was not something they could face on their own and the other thing that they did not know about, was the fact that even if they somehow defeated this creature, it would regenerate to its current form instantly.
