Diablo is the final boss of Diablo I.
Diablo's lair can be entered by passing through the opening in the pentagram on level 15. A special loading screen will appear and you'll find yourself on the lowest level of Hell, Level 16. This final level is filled with Blood Knights and Advocates, the most vicious hellspawn in the game, so be prepared to cut your way through many monsters. In each corner of the room is an area with a special "Skull Lever" which you have to activate. At first only one room is accessible, but as you activate the lever the next room opens up. The third room contains two levers; after activating both the fourth and last room opens... and Diablo, whose title in the game is simply "The Dark Lord," awaits.
There is a relatively easy way to defeat Diablo. Although it works best for the Sorcerer, other classes can try it too. You only need a fairly large manapool, loads of Mana Potions and the Mana Shield and Fire Wall spells (higher levels work better). After Diablo has been released, lure him to a relatively safe area where you can fight him alone without additional monsters. Make sure you have Mana Shield up, allow Diablo to walk towards you and cast Firewall on him. Diablo will frantically cast Apocalypse on you, but due to mana shield you will only lose mana instead of life (Sorcerers are most likely to have a huge mana pool. Warriors can try to either soak up the damage with their inherent vitality and/or health potions, or even block the attacks if they wield a shield; in that case they don't need mana shield. A Chance To Hit of 120%, a weapon with an attack speed suffix, and minimum points of 30 damage and maximum of 50 or over will assure that Warriors can give multiple successive blows on Diablo, making him rather helpless). Make sure a Fire Wall is up at all times, refill your manapool with potions in order to prevent losing health. After some time Diablo will start casting Apocalypse even faster in a desperate attempt to save himself, just keep drinking mana bottles. After some more seconds of roasting over the fire, Diablo will fall.
An alternative, possibly even faster method would be using holy bolt spells. Even though the spell's description does not mention demons as being affected, it is possible to kill Diablo rather quickly with a sustained barrage of Holy Bolts. This strategy is not really recommended for Nightmare or Hell difficulty, however, especially in multiplayer; on Hell difficulty, he could take well over 100 Holy Bolts before succumbing to death. That is well over at least 300 mana, which is definitely not easily done with Warriors, if at all possible.
When a character defeats Diablo, they gain 37,999 experience points. Diablo is resistant to Fire and Lightning, and immune to Magic (though some spells can still hurt him). He has 1,666 hit points on Normal difficulty.
In the Anniversary Dungeon of Diablo III, players can fight the Dark Lord again. The arena is laid out much like the original, where you have to pull levers to open the next door and release a horde of monsters. Pulling the final one will open Diablo's cage and release the Dark Lord's fury.
Upon death, he drops Red Soul Shard and concludes the dungeon. The classic ending cinematic will be played upon killing him.
- "Eat your vegetables, and brush after every meal!" (Said in reverse)