The setting of Diablo is the town of Tristram, the de facto capital of the Kingdom of Khanduras on the world of Sanctuary. The actual fighting takes place beneath the town in a maze of dungeons, catacombs, and caves that lead into the depths of Hell.
The plot of the original Diablo game centers around a player character undertaking a series of quests to free Tristram from Hell-spawned evil, descending through twelve levels of dungeons, catacombs, and caves into Hell itself (the final four levels), where the player battles the title character, Diablo, Lord of Terror — one of the seven "Evils" (devils) who once ruled Hell.
At the end of the first game' date=' a warrior tried to contain Diablo's soul within himself. The warrior was unable to do so, and by the beginning of Diablo II, The Lord of Terror has taken control of this warrior's body and begun the process of freeing his two brothers, Mephisto and Baal. Players can choose from five distinct characters (seven when including the expansion) to control in their quest, and explore the world of Sanctuary through four acts. At the end of each of the four acts, players face different devils, with Diablo at the end of the game.[/quote']
But in general...bad evil demons from hell invade the mortal world. You kill demons and take their gold and loot.