Diablo
Diablo is an awesome trinity of dark fantasy dungeon-crawling games from Blizzard. Set in the world of Sanctury, they tell of the ongoing struggles of humanity against the backdrop of the eternal war between the High Heavens and the Burning Hells, which has a tendency to spill into the mortal world. All three are some of the best examples of old-school "are you a bad enough dude to fight your way through the dungeon of evil?" hack-slash gaming you'll find outside of the tabletop.
Basic Backstory
In the beginning, there was the emptiness of creation, inhabited only by Anu, the Spirit of All. In pursuit of true perfection, Anu cast out those aspects of itself that embodied concepts of evil, only for them to become incarnate on their own as a monstrous seven-headed dragon called Tathamet, the Prime Evil. The two spirits fought to the death and ripped each other apart, creating the universe. Most notably, Anu's spine became the bedrock around which formed the High Heavens, whilst Tathamet's festering corpse becomes the Burning Hells.
From Anu's spine, now known as the Crystal Arch, were born first five Archangels, representing the Five Virtues of Valor, Justice, Hope, Fate and Wisdom, and then legions of angels.
From Tathamet's corpse, demons arose like monstrous maggots, whilst the dragon-god's decaying heads became mighty arch-demons; the three Prime Evils (Mephisto of Hatred, Baal of Destruction, Diablo of Terror) and the four Lesser Evils (Durial of Pain, Andariel of Anguish, Azmodan of Sin and Belial of Lies).
Naturally, the two races took one look at each other and instantly wanted to eradicate each other.
For untold eons, angels and demons butchered each other, until a small band of rebels and dissidents grew in the ranks of each side. Led by the angel Inarius and the she-demon Lilith, these dissidents banded together and fled the war, capturing Anu's crystalized eye - the Worldstone - and using it to forge a world in which they could hide from their fellows, a world they called Sanctuary. In time, they interbred, creating a new race of powerful beings called the Nephalem.
Unfortunately, this peace couldn't last forever; Sanctuary was discovered and the progenitors destroyed, whilst angels and demons then tried to figure out what to do with their bastard descendants. Through a convoluted sequence of events, involving one Nephalem using the Worldstone to bind the powers of his kind, reducing them all to humans, the two races agreed to leave Sanctuary to its own devices and turned back to their war.
Until, one day, the Lesser Evils grew sick of the dominance of the Prime Evils, and banished them to Sanctuary. There, the Archangel of Justice, Tyrael, allied with a group of mystics and sages called the Horadrim, who defeated the Prime Evils and bound them into magical crystal prisons called the soulstones. Naturally, the Prime Evils want out... which doesn't spell anything good for Sanctuary.
The First Game
The soulstone of Diablo, Lord of Terror, came to rest in a labyrinth hidden beneath the church of a town called Tristram in the land of Khanduras. The corrupt Archbishop Lazarus retrieved the soulstone and unleashed the weakened Prime Evil's essence into King Leoric of Khanduras, driving the king into madness and evil before ultimately abandoning his body to possess and subvert that of Leoric's younger son, Prince Albrecht. Leoric was ultimately slain by Lachdannan, one of his own knights, but too late; Diablo now roamed the labyrinth below Tristram, allowing the hordes of the Burning Hells to begin leaking through and ravage the countryside above.
A player takes the role of one of three mighty adventurers who have come to investigate the darkness plaguing Tristram, with a single, simple goal: butcher their way to the dungeon's heart and kill Diablo.
Diablo Classes
As this was the first game, only three classes were presented as player options. This being early days, they were much less defined then they would be in later games. Though their strengths and weaknesses affected what they could do, theoretically, any class could be pushed into any direction; melee, ranged or magic.
The Warrior is your basic Fighter type. With the highest Strength and Vitality scores in the game, it's a classic tank, excelling at wearing heavy weapon and using powerful weapons to crush and slaughter whatever stands in its way. It's the easiest class to play as, given it's your standard "get up close and hit things" character. His unique Skill is Repair, which lets him fix up damaged items at the cost of permanently lowering their maximum durability, allowing them to prolong their usefulness. Canonically, the Warrior is Prince Aidan, elder son of King Leoric.
The Rogue is the only female character, and hails from the Sisterhood of the Sightless Eye, an order of female warrior-women that splintered off/descended from the Amazon warriors of the Askari. Consequently, the Rogue plays much like a precursor to the Amazon class in the second game. Rogues have the highest Dexterity in the game, start with a bow and have the unique ability to fire bows faster than either of the other classes. As a result, they're optimised for ranged combat, specializing in shooting demons down from afar and running away from melee fights, though they're better at magic use than the Warror is. Her unique Skill is Trapfinding, allowing a player a chance to avoid those pesky exploding barrels and projectile-launching chests. The canonical name of the Rogue is Moreina.
The Sorcerer is your magic-using class, capable of casting spells faster than any other and with the greatest Magic score in the game. The Sorcerer is the frailest character in the game, with the lowest Vitality score, but his powerful spells make up for it quite amply. It does make lower levels much trickier, as he will spend a lot of gold of health and mana potions to keep himself going. His unique skill is Recharge Staves, which allows him to re-imbue magic into staves so he can keep casting spells from them, though like the Warrior's Repair skill this does permanently lower the amount of magic a staff can store, until it becomes useless. The Sorcerer hails from the Vizjeri mage clan, the most infamous clan due to its long association with dark magics - they were the first mage clan to ever try and magically control demons. The canonical name of the Sorcerer is Jazreth.
The Second Game
Although Diablo was slain beneath Tristram, it wasn't without cost. Prince Aidan thrust Diablo's soulstone into his body in an attempt to contain the Lord of Terror, but was ultimately consumed from within; as the demon-summoning Dark Wanderer, he sets off to recover the soulstones of the other Prime Evils, allowing them to be free to crush Sanctuary and then return to the Burning Hells. Neither of the other adventurers from Tristram fared any better; Moreina was slain by the Lesser Evil Andarial, Maiden of Anguish, and resurrected as a demonic undead creature called Blood Raven, whilst Jazreth lost his mind and became a demon-binding mystic called the Summoner.
The player takes the role of one of the five classes, attempting to catch up to the Dark Wanderer and stop his evil plans, with designs of ultimately slaying Diablo and Mephisto alike by invading the Burning Hells and shattering their soulstones upon a mystical anvil. They succeed, but Baal remains free... until the expansion pack, Lord of Destruction, was released, which adds two new classes (who can fight through the entire game) and revolves around travelling to Mount Arreat to stop Baal before he can take possession of the Worldstone.
Wizards of the Coast liked this game so much that they did up a splatbook to play in the Diablo world using Dungeons & Dragons 3rd edition rules, the now-obscure but pretty faithful Diablo II: Diablerie. Sadly, it came out before Lord of Destruction, so it only features the base Diablo II classes. Still, they work fairly well under the 3e rules, and manage to be surprisingly faithful to the actual game.
Diablo II Classes
Diablo II set new ground for Diablo by coming up with a new mechanic; skill trees. Each class has three unique sets of special talents that it can develop as it gains levels, making them far more flexible, whilst still having a coherent structure that made it easier to build around than the original game's Warrior/Rogue/Sorcerer trinity.
The Amazon hails from the warrior-caste of the matriarchal jungle culture of the Askari. As mentioned above, she builds on the talents of the Rogue and is a primarily ranged-focused fighter. Her skill trees are Bow & Crossbow (trick shots like fiery arrows, freezing arrows, homing arrows, etc), Passive & Magic (basic buffs like increased chance of criticals and spell-like effects like illusionary doubles to draw aggro), and Javelin & Spear (melee attacks and special projectiles).
The Assassin is a female warrior from the Viz-Jaq'tar, a martial order of elite wizard-killers specifically founded and trained by the Vizjerei Mage Clan in order to police the Mage Clans and ensure that they never fall into demonic corruption again. They favor claw weapons, wielding esoteric martial arts and deploying lethal enchanted traps in their duties. An assassin is a quick-moving, quick-hitting character that favors hit-and-run tactics. Her Martial Arts skill tree increases her lethality in melee, whilst Shadow Disciplines enhance her abilities to escape from danger and Traps, of course, allows her to leave all kinds of nasty surprises for anyone trying to follow her. They're one of the two classes added by the Lord of Destruction expansion pack.
The Barbarian hails from the lands surrounding Mount Arreat, where tribes of these savage warriors have defended the Worldstone for countless generations. One of the two melee-focused classes, alongside the Paladin, the Barbarian is a killing machine, ploughing into the ranks of the enemy and leaving only piles of corpses in his wake. So not too dissimilar from Dungeons & Dragons, then. His skill trees are all straight-forward; Combat covers special attacks like Bash, Combat Masteries is an array of permanent passive buffs, and Battlecries act to temporarily buff the Barbarian and allies and debuff enemies.
The Druid hails from the lands of Scosglen (yes, we winced too) and is a distant cousin to the Barbarian, historically and culturally. Although magic users, the druids stand apart from the Mage Clans, forsaking the use of sorcery in favor of their own brand of nature-magic. They're pretty close to their Dungeons & Dragons cousins, being capable melee fighters, potent spellcasters and skilled summoners all at once. Their Elemental skill tree covers basic attack spells, like Firestorm and Tornado. Their Shapeshifting skill tree lets them turn into a werewolf or werebear and beefs up their skills in said forms. Their Summoning skill tree lets them call animals, killer plants and even nature spirits to fight alongside them. They're one of the two classes added by the Lord of Destruction expansion pack.
The Necromancer is not what you'd expect in a game like this. The Necromancers of Sanctuary are actually Wizard-Priests dedicated to honoring and upholding the Balance, a mystical philosophy developed by their founder, Rathma, and embodied in the dracolich pseudo-deity Trag'Oul which emphasizes the importance of equilibrium between life and death, as well as light and dark. Consequently, necromancers stand opposed to angels and demons alike, judging both to be dangerous to the existence of humanity and Sanctuary as a whole. The Priest of Rathma is akin to the Sorceress in being a caster class, but is a minion-master in contrast to the Sorceress' "blow shit up" focus. His skill trees consist of Summoning (minion-making), Bone & Poison (offensive and defensive spells) and Curses (debuffs).
The Paladin is a worshipper of the Zakarum church, a holy warrior who has managed to avoid succumbing to the rot that has eaten his religion out from the inside over centuries of guarding the soulstone of Mephisto, Lord of Hatred. A melee-focused class, the Paladin is a classic tank, specializing more in soaking up damage than in brutally hewing through enemies. Its magic manifests mostly in the forms of holy auras, both Offensive and Defensive - this means Paladins excel when they can fight in groups, as they naturally buff the rest of the party. Their Combat Skills tree covers various holy magic spells and attacks, such as their iconic Holy Bolt and Smite powers.
The Sorceress follows in the wake of the Sorcerer from the first game. Hailing from the Zann Esu clan, a matriarchal (women-only) Mage Clan dedicated to the mastery of Elemental magic, she specialises in blasting the hell out of enemies with Fire, Ice and/or Lightning magic.
The Third Game
Baal, Lord of Destruction, was destroyed, but so was the Worldstone to save it from his corruption. Now, a decade later, the last remaining Lesser Evils, Azmodan and Belial, are stirring, attempting to crush Sanctuary once more. A new brand of heroes must arise, fighting all the way to the High Heavens themselves, where they face the ultimate foe; the True Evil, the collective manifestation of all Seven Evils, reborn into a strangely creepy-sexy feminine form.
It built upon efforts from Diablo II, attempting to improve them. For the first time, you could choose the gender of your PC, instead of having it determined by your class. You also get three unique followers, allowing you to make your own adventuring party, rather than the generic mercenary hirelings of the previous game.
Like with D2, it got an expansion pack. Called Reaper of Souls, it revolves around Maltheal, Archangel of Wisdom, being corrupted into the Archangel of Death and attempting to wipe out humanity, proving once and for all that even the angels are no better than the demons in this world.
The game got large amounts of flak from players (including Blizzard fans) for how much of the content that made Diablo II was cut out and many of the things simplified. No wonder, since Diablo III was made by the same people behind World of WarCraft. Not even Reaper of Souls fixed it, ironically making Diablo III the least Diablo-ish game in the series...not to mention the plot being really bad along with the PCs and NPCs having bad dialog.
Diablo III Classes
The classes in Diablo 3 have a very different setup for their skill trees than those in Diablo 2. They also have more fleshed characterization since they talk a lot more, contrasting in D2 where they only spoke a few times in the whole game.
The Barbarian is a survivor from the now-scattered tribes that once guarded Mount Arreat. Whereas many have fallen into despair and have forgotten their traditions, he or she still clings to them. The Barbarian is still a brutal killing machine, retaining many of its skills from the last game and specializing in getting into the fray quickly and chopping down whatever stands in its way.
The Crusader is an heir, thematically and in-universe, to the fallen Paladins of Zakarum. A tank's tank, capable of dual-wielding greatweapons or carrying a greatweapon and a shield, it's a heavily armored juggernaut that takes a beating and shrugs it off, all while bolstering itself and smiting foes with holy magic. It was added to the game with Reaper of Souls.
The Demon Hunter is a scarred vigilante who burns with one goal; the extermination of all minions of the Burning Hells. Mechanically, it owes its lineage to both the the Amazon, and Assassin of the last game, with a focus on ranged combat, traps and evasion.
The Monk hails from Ivgorod, Land of 1000 Gods, which is sort of Russia meets China. Devoted to mystical martial arts, it proves quite adept at beating evil to death with its bare hands. Mechanically, it owes its lineage to the Assassin, specializing in agility and quick strikes.
The Witch Doctor is a member of the jungle-dwelling Umbaru tribes, using its connection to the spirit world to cleanse the land of evil. It shares thematic ties with the Necromancer and the Druid, being a non-member of the Mage Clans, but is closer to the Necromancer in terms of mechanics, with a focus on creepy summons and damage over time.
The Wizard is a voluntary outcast from the Mage Clans, forsaking the traditional methods of sorcery for more potent and dangerous spells, allowing it to wield raw arcane force and to control space and time. As you'd expect, it is heir to the throne of Blowsshitupistan, inherited from the Sorcerer and Sorceress.