A normal dungeon?

People were going about their days even though dungeons could appear anywhere, anytime, safely, thanks to the safety measures of adventurers and mercenaries. However, one day, a strangely large and more menacing dungeon appeared. People thought it was just a normal dungeon and walked past it. A few days passed, and it was unusual that the dungeon hadn't been closed yet. After another day passed, rumors spread that all the adventurers who entered the dungeon hadn't come out and were presumed dead.

Following that incident, they closed the vicinity of the dungeon to the public. Six months later, they attempted another raid on the dungeon with stronger adventurers, but it also failed, and they didn't return. They conducted a total of four raids, all of which failed.  As time passed, the mysterious, ominous dungeon, its dark presence damaging the surrounding area.  speculations spread among the people, and they started to making rumours of curses and ancient evils hidden within its depths.

The local authorities, desperate to resolve the issue and restore peace, they tried asking  wise sages that lived in mountains, hoping to uncover any clues or ancient texts that might help them close the dungeon.Despite their efforts, there was no knowledge of that dungeon.  

 In Present, the area around the impenetrable dungeon stands desolate.