Dungeon Busters : I Am Just Middle-Aged Man, But I Save the World Because of Appeared the Dungeon in My Home Garden

Clear all the dungeons! There are 666 of them and 10 years to do it. Otherwise, the only thing waiting...is complete annihilation. Ezoe Kazuhiko suddenly found a mysterious dungeon in his yard. When he stepped inside to see what was going on, it turned out to be the world's first dungeon, and in the end he accidentally activated the System.