Demon King Cheat System is a Manga/Manhwa/Manhua in english language, Manhua series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

One day, countless dungeons suddenly appeared in the world, and human beings ushered into the “dungeon era”. The boss of the abyss-level dungeon, Ah de sa lai, was killed by human players using despicable means. Unwilling to accept it, he was reincarnated into A weak and also useless player accidentally, opened the exclusive boss system, using the system to conquer the dungeon and to gain the special ability of the dungeon master, he started his own road of revenge~