Diablo is an action role-playing hack and slash video game. The player moves and interacts with the environment primarily by way of a mouse. Other actions, such as casting a spell, are performed in response to keyboard inputs. The player can acquire items, learn spells, defeat enemies, and interact with non-player characters (NPCs) throughout the game.

The dungeon levels are procedurally generated with themes for each level; for instance, the catacombs tend to have long corridors and closed rooms, while the caves are more non-linear. The players are assigned a random number of quests from several tiers; these quests are optional but usually offer powerful unique items as rewards and help to level up the character and/or reveal more of the backstory. The final two quests are mandatory in order to finish the game.

The setting of Diablo includes the mortal realm which is the world of Man as well as the High Heavens and the Burning Hells. After eons of war between angels and demons, the ascension of man prompted the three Lords of Hell (including Diablo himself) to seek victory through influence, prompting their exile into the mortal realm. There, they sowed chaos, distrust, and hatred among men until a group of magi, called the Horadrim, trapped them in enchanted crystals called “Soulstones”. Diablo’s soulstone was buried deep in the earth and a monastery was built over the site.

Generations passed and the purpose of the monastery was forgotten. A small town named Tristram sprang up next to the monastery’s ruins. When King Leoric rebuilt the monastery as a cathedral, Diablo manipulated its archbishop, Lazarus, to destroy his soulstone prison. Diablo briefly possessed the king, sending out his knights and priests to battle against peaceful kingdoms, and then possessed the king’s son, Prince Albrecht, filling the caves and catacombs beneath the cathedral with creatures formed from Albrecht’s nightmares.

Tristram became a town of fear and horror, where people were abducted in the night. With no king, no law, and no army left to defend them, many villagers fled.