Set in the fictional Kingdom of Khanduras, located in the world of Sanctuary, Diablo has the player take control of a lone hero battling to rid the world of Diablo, the Lord of Terror. Beneath the town of Tristram, the player journeys through sixteen dungeon levels, ultimately entering Hell itself in order to face Diablo.
Diablo is one of the best-known examples of the action-RPG subgenre. Although players level up, choose character classes, and manage a variety of spells and equipment as in a typical RPG, all actions are done in real time, as in an action game. Diablo in some ways resembles roguelike games, though major differences include the commercial quality of the game's graphics, the fact that it plays in real time rather than as turn-based, that games can be restored after the player dies where most roguelikes feature permadeath, and Diablo's relatively short learning curve. Diablo was influenced by Moria and Angband. The majority of commands in Diablo, such as moving and attacking, are executed by mouse clicks; however, learned spells can be assigned hot keys.
Gameplay is structured around a monster-filled dungeon located near the town of Tristram. In town, players may rest, buy items, and repair equipment. There are sixteen levels of the dungeon, divided into four areas. Each area has a different appearance, architecture, light level, monster mix, and musical soundtrack. The first level of each of these areas has an additional exit leading back up to the town of Tristram. In single player, these entrances are blocked until the character opens them from the dungeon side, and the entrance is available for two-way travel from then on. In multiplayer, the entrances to town all start in their "open" position, but with a level requirement to access them from town.
Diablo is highly re-playable due to its randomly generated level layouts, monsters, and items. In addition, in single-player mode there are only three core quests as the rest of them are drawn from several pools, making it impossible to complete every quest in one playthrough of the game. Either way, only the last two quests are compulsory. Given this arrangement, no two playthroughs of the game are ever exactly alike.
The three character classes of Diablo are the warrior, rogue, and sorcerer. Each character, following typical role-playing conventions, has his or her own particular traits. The warrior possesses physical strength, the rogue has high dexterity, and the sorcerer is oriented towards magic.
- Warrior: The warrior is a powerful melee fighter, master of weapons of war and capable of enduring more damage than the other classes. They range from barbarians of the northern highlands to noble paladins. The voiceover for this character, as well as for Pepin the Healer and narrator, is performed by Paul Eiding.
- Rogue: The rogues are the best archers in the world of Sanctuary. They can have a higher level of magic than warriors, although not nearly as high as sorcerers. The rogues belong to a group called the Sisterhood of the Sightless Eye. The voiceover for this character, along with Gillian the bar maid, is performed by Glynnis Talken Campbell.
- Sorcerer: A powerful master of the arcane arts, the sorcerer is able to achieve the greatest heights of magic, so that he doesn't have need of physical weapons. Sorcerers belong to the Vizjerei mage clan, and have come to Tristram seeking long-lost tomes of magic knowledge under the cathedral. The voiceover for this character, along with Cain the elder, is performed by Michael Gough.
Unlike other games that strictly differentiate between classes, a character's abilities are not unique; a warrior can use the same spells as a sorcerer, while a sorcerer can use weapons such as axes. All three classes require the same amount of experience to level up, and there are no class-based requirements for equipping items or using spells. However, different classes have different starting attributes and different maximum possible levels for their attributes, and gain different amounts of life and mana per level.
Monsters and items
Each of the sixteen levels in Diablo contains monsters that are tougher and stronger than ones from the level before it. When the player kills a monster, it may randomly drop an item or gold. Upon killing more enemies of the same type, the player may find out more details about the monsters, such as hit points and resistances or immunities. Enemies are divided in three groups: Animals, Demons, and Undead. The group determines which weapons the enemy takes more or less damage from.
Items are sold by the vendors, randomly dropped by slain monsters, and can be discovered within the labyrinth inside of chests or barrels or sometimes lying on the floor. There are several types of items. Gold is the currency used to buy goods and services from the vendors. Consumables are items that are destroyed when used, and include life and mana potions, elixirs to increase base attributes, scrolls to cast spells and spellbooks to learn spells permanently. The player has eight slots representing a belt which can contain only consumables (except spellbooks). These slots are numbered, and pressing the corresponding hotkey will use the associated consumable. A special kind of items are quest items, which come in many varieties. Some of them activate a quest when picked up or found, while others must be carried along or used to interact with the environment, and yet others are given as special rewards for completing quests.
Any character can use any piece of equipment so long as they meet its statistical requirements (Strength, Dexterity, and Magic). Weapons and protective gear have durability values that decrease with use, but can be restored through several means. If durability reaches zero, the item is destroyed. Staves are two-handed weapons used primarily for the spell charges they contain, which can be recharged. Each charge allows one casting of the spell contained within the staff.
Diablo helped popularize a system used in other RPGs such as the Might and Magic series, to handle the many combinations of random items imbued with random magical properties. Magical items in Diablo have an idiosyncratic naming system; a particular enchantment will be either a suffix or prefix. For example, the "Godly" prefix, appearing only on armor, adds greatly to armor class. An item with this ability would appear as "Godly (itemname)". Magical items can have both a prefix and a suffix; however, certain systemic limitations within the game mechanism prevent some prefixes and suffixes from appearing together on the same item. Different equipment types draw from different pools of affixes; some affixes are never available on certain types of equipment.
Equippable items can have various modifiers, and break down into three major classes: normal items (items that have no special attributes and are most abundant), magic items (that can have up to one prefix and one suffix) and unique items (very rare and powerful, and may have up to six magic bonuses). Magic and unique items must be identified before their modifications become known.