Kaizo Trap: Elite Mooks with the "Molten" ability will leave behind a fireball when they die, which explodes in a radius for massive damage. Better not pick up that loot yet...
Karma Houdini: Adria, who betrays you and resurrects Diablo by sacrificing Leah, manages to escape. This state of affairs continues until Reaper of Souls, where she shows up as Act V's second major boss, allowing you to finally give her the asskicking you've been wanting to give her since her betrayal at the end of Act III.
Kick the Dog: As you close in on Diablo in the final act, he uses the apparitions of those who died during (or even before) the game to taunt you. The very first one he uses? Leah.
The very first thing you see a Death Maiden doing in Reaper of Souls is ripping out the soul of an innocent woman trying to escape Westmarch and turning her and the dead around her into Reapers to send after you, with the cold "All with demon blood must die." It quite nicely sets the tone of both the expansion and the Reapers' mission.
Killed Mid-Sentence: Can be invoked by the player. While the subtitles will show the entirety of any mook banter, if you kill the speaking mob before the audio of the banter has finished, the audio will cut off.
Despite appearances, Leah is not necessarily an example of this. Covetous Shen, who's possibly a god, and even if he's not is very old and has possibly seen more than even Deckard Cain or Tyrael suggests she may have survived in some form, and unlike other times Diablo is defeated, her corpse isn't left behind. On top of that, Blizzard has teased her resurrection as a plot point for the expansion(s).
Killer Rabbit: The enemies in Whimsyshire. Cute unicorns, teddy bears and pink flowers that will hand you your ass quickly if you're not careful. They're also the highest-leveled enemies in each difficulty level.
Kleptomaniac Hero: The game lampshades this: if Leah is in your party and you enter her room at the inn, she will react uncomfortably. She'll be downright appalled if you read her journal, which is an absurdly oversized book sitting on her desk. (Journals and correspondence can often be found on corpses and among people's possessions, and contain material which, while not usually indispensable to gameplay, usually deepens the plot. Also, there are achievements based on reading enough of them.) She'll also object if you take her into Cain's house while he isn't home.
Knife Nut: The lowest ranks of the cultists merely run toward the player character brandishing daggers.
Angels as a whole can be considered this as well, especially Imperius.
Lady-In-Waiting: Queen Asylla's handmaidens were slaughtered during Leoric's madness and rose as the Wretched Mothers.
Ladykiller in Love: Part of the backstory of The Rogue, Lyndon. Exactly how much SUCCESS he has with women is anybody's guess, but he's definitely eager to flirt with anything in a skirt... or a fur bikini... or a skimpy mage-robe... he's not picky, really, and doesn't even wince at being shot down. If you chastise him for his womanizing, he claims that he's only ever met one woman 'worth keeping'. Further digging into his past reveals that he originally turned to crime in order to fund his attempts to woo her, and that she wound up marrying his goody two-shoes brother instead. The pain in his voice is audible whenever she comes up in conversation, turning his otherwise Loveable Rogue demeanor bitter in a split second...
While not strictly land mines, the desert has venomous plants that explode once you get near, after giving a warning.
Throughout Arreat Crater and its associated subzones.
Pandemonium Realm is littered with mines which, when triggered, beep in escalating frequency before exploding.
Last Chance Hit Point: Every class has an available passive skill that saves the hero if they take mortal damage, through different means such as knockback or Mercy Invincibility. These skills have a 60-second cooldown, so if you can't get away as soon as they activate, you're toast.
Lawful Stupid: This is how Tyrael sees the Angiris Council; because of an ancient pact and having an ancient scroll which they believe to be able to foretell everything, they will not interfere and thus put both themselves and humanity in danger. That is why he became a mortal, willingly.
Legendary in the Sequel: The game has numerous references to the heroes from Diablo II. Most of them are generic mentions of a "band of heroes" in Deckard Cain's journals. The most specific it ever gets is when you meet a necromancer who says his mentor helped defeat the Prime Evils twenty years ago.
Lethal Lava Land: The game has lava landscaping in several non-Hell areas, including Leoric's dungeons and the lower levels of Bastion's Keep. You can't fall into the pits, but a typical trap has periodically-erupting lava underneath a grate floor. Don't be standing there when it goes off.
Level-Locked Loot: The game has "reduced level requirement" as a possible random ability on some gear, making level 60 kit that you can wield at (say) level 46 that still has the stats of level 60. The gap between 46 and 60 is a lot bigger than that between 32 and 46...
Level Up Fill Up: Both your health and mana is restored on leveling. This game even takes this further where you shoot off an aura that damages your enemies on Level-up.
Lighter and Softer: The somewhat more saturated palette of this game compared to its predecessors elicited accusations of this from the fans. Blizzard decided to mock the accusers back by releasing a shirt looking like this◊, and then going even further and creating a level in the actual game with the same theme - see the Sugar Apocalypse entry.
Light Is Not Good: Imperius embodies this trope, not caring about mortals in the slightest. He was the main agitator among the Council for extermination of humanity, and since Malthael, the Aspect of Wisdom, was absent from the Council, his vote was counted against humanity in absentia, with Auriel and Itherael voting for humanity's survival. Tyrael's final vote thus saved humanity from annihilation.
Lightning Bruiser: The Barbarian can become one with the speed boost blessing, especially if he/she already has movement speed boosts.
Limited Wardrobe: The Blacksmith and the Jeweller, Covetous Shen. Especially noticable with the latter, as he still wears his desert-faring clothes in the cold Bastion's Keep. The player can question him on this after Shen advises him to wrap up warm, and he instantly lampshades and justifies it.
Locked In The Dungeon: While the game involves plenty of dungeon-crawling, the prime example of this trope occurs in the final part of Act I, where you have to storm the Halls of Agony, King Leoric's old torture chambers, in order to find and rescue the Stranger you found at the impact zone of the Fallen Star, who has been taken there by Maghda and her Dark Coven. Since this is the Diablo universe, the Coven has turned the Halls into a site for Human Sacrifice and Cold-Blooded Torture of their many victims, with all the horrificness that this implies, and the place is loaded with undead and demons for you to kill in addition to the cultists, as well as the ghosts of those who were put through hell and executed down there back when Leoric was still alive and insane.
Loincloth: Among the PC classes, the Witch Doctor's attire generally includes a loincloth, and the Barbarian's starting outfit is little else. Many NPCs wear loincloths as well (especially the demons).
Lost Forever: The developers are considering introducing a setpiece revolving around a collapsing dungeon full of treasure that players have to make a decision on whether to run and save themselves or collect the loot and potentially die. Given the random nature of the game, whatever items are in the dungeon will probably be lost if not picked up then. Please, somebody stop them.
Considering that all loot in the Diablo series is randomly generated, any items that get lost when the tomb collapses could probably be found anywhere else. Based on experience with the actual dungeon at Blizzcon, the true value was that the tomb was extremely abundant in Resplendant Chests (like, 2 in every room), which are guaranteed to drop a rare item and 2-5 magic items. Plus everything resets when you quit and come back, so you have a chance to go again if the entrance randomly appears.
In the majority of Hack and Slash games with a Random Number God, any piece of gear with a specific combination of stats can be considered Lost Forever if the player fails to notice it while sifting through Vendor Trash or accidentally gets rid of it, due to the chances of the same stat rolls lining up being less than one-in-a-million. Wonder why hacking is so attractive in online play?
Love at First Sight: If Kormac the Templar is your follower the first time you encounter Eirena the Enchantress, he will remark that she is "beautiful." In subsequent conversations between the two characters it's evident that Kormac is interested in her romantically.
Luck Stat: Magic Find, Gold Find, and Critical Hit Chance.
Every enemy in the game can be exploded or dismembered in some way. Any enemy killed with a critical hit will explode (and the gibs themselves will be on fire/frozen/glowing with magic energy depending on damage type), all Unique monsters will explode when killed, some breeds of monsters explode no matter what... etc. This feature was so popular that shortly after the game's unveiling, Blizzard gave in to fan's demand that corpses stop fading away, just so they could see the aftermath.
The Monk has a technique called Exploding Palm - enemies struck by this explode when killed by DOT. Omae wa mo shinderu...
Lull Destruction: The followers will invoke this by jumping into random conversation every minute or so in the middle of battle, and complaining loudly if you stand still for about half a minute. Some of these talks establish character and plot, but many are reused from act to act.
MacGuffin: The game, of course, continues the trend.
Act 1, To enter the lower levels of the cathedral, find Leoric's MacGuffin in one of the cemetery crypts. Later on, send the player off to locate the three shattered sword pieces of an angel's MacGuffin, but wait... to gain access to one of the rooms that a sword piece is in, you need to side-track and find the two Orbs that will open the door in the nearby forest.
Act 2, Shen's introduction has him finding an item. Then of course there's Zoltan Kulle's long side-tracking task of finding the pieces of his body that must then be put together in his hidden library to make a full MacGuffin. The Act then introduces the Black Soulstone MacGuffin which apparently houses all the souls of the defeated evils from Diablo II. Belial's MacGuffin is then placed in it at Act's end.
Act 3, The Black Soulstone is seen throughout the Act. Once Azmodan's MacGuffin is placed in it at Act's end, the plot behind the Black Soulstone will then start-up the events for the next Act.
Act 4, Diablo itself, is the MacGuffin; being the second rendition of the Black Soulstone.
Guard 1: I just found a dusty wine bottle stashed in a hole in the wall. Guard 2: This place was built by Kagus Deel, famed drunken architect. He has stashes all over the keep.
Mana: The game has permutations based on the class.
Barbarians build Fury with several of their basic and combat-initiating abilities, and also by taking damage. Fury can be expended in much more powerful skills, though a passive ability can be chosen to grant highly increased damage with a maxed-out Fury orb.
The Demon Hunter is unique in using two resources for their skills. Hatred fuels offensive abilities and quickly replenishes, but a more limited and slowly regenerating pool of Discipline is required to use vital defensive and tactical skills.
Monks gain holy Spirit through their combo attacks. Spirit does not decay over time, and the Monk may choose when and how to expend it with his myriad of offensive and supportive abilities.
Witch Doctors continue to use the Mana Meter, though its regeneration rate is relatively higher than most other games. Mana-siphoning abilities and "investing" mana into Summon Magic help to keep the Witch Doctor casting.
A Wizard channels a constant flow of Arcane Power into their spells. Their most powerful abilities can consume nearly the entire pool at once, though Wizards also have costless "Signature" abilities to deal moderate damage in the meantime.
Many Spirits Inside of One: The Prime Evil (as opposed to the Prime Evils, which are merely the three most powerful of the Great Evils) is the embodiment of all seven Great Evils in one being. The original Prime Evil was Tathamet, from whose body the Burning Hells and the original Great Evils sprang, but in this game, Diablo is reborn as the Prime Evil, and the reason he is able to be dominant over his brothers in this form is because Adria used Leah, her own daughter by him by means of his last host, as his vessel.
Meaningful Background Event: In Acts III and IV you can see defenders fighting demons in the distance; in particular, in Act III the Siegebreaker Assault Beast is very prominent, laying waste to soldiers, so it's no surprise when you get to Azmodan's front lines and fight him.
Meaningful Name: The third game introduces Imperius the Archangel of Valor. "Imperius" is clearly a differently spelled "imperious", which means "domineering, overbearing, arrogant; urgent; (obsolete meaning) imperial or regal". Interestingly enough, "valor" means "value; worth; strength of mind in regard to danger; that quality which enables a person to encounter danger with firmness; personal bravery; courage; prowess; intrepidity; a brave man; a man of valour; arrogance". When you think about how Imperius tried to stop Diablo all by himself, and that he seems to be unbelievably arrogant, well, his name and title says it all!
Mercy Kill: Early on, you have to help New Tristram's blacksmith, Haedrig Eamon, kill his wife Mira and everyone else who's been bitten by the zombies attacking the town. The page quote is the Demon Hunter's response to Haedrig's question about how he could kill his own wife. Later on in the game, when Leah is possessed by Diablo himself following Adria's betrayal, the final battle is essentially this for Leah as well as a final reckoning for the Prime Evil.
Metal Slime: Treasure goblins drop a lot of gold and items but flee from the player and if not killed in short order will disappear through a portal. Although they don't attack, following them can be dangerous if they lead you to a giant pack of hungry monsters.
Meta Power-Up: The game has some very high level items that increase XP gain, and items ranging from low to high levels that increase gold drops or the chance that randomly dropped items will be magical (and the power of magical items that drop). There are also similar items at lower level. +1-2 XP per kill, +x% chance of magic drops, more gold.
Mighty Glacier: The Crusader introduced in Reaper of Souls has a passive ability that lets them equip a two-handed weapon in one hand at the cost of 15% of his movement speed.
The Minion Master: Witch Doctors are capable of summoning various creatures alongside them.
Monks get the Mystic Ally ability, which summons what is basically an ethereal version of themselves that, just like the Witch Doctor's minions, stays summoned until it is destroyed.
Mistaken for Subculture: When the secret level turned out to be full of purple unicorns, many assumed it was inspired by the Brony subculture. It was actually the result of an early controversy regarding "too bright" color design in screenshots, mocked in thisPenny Arcade strip.
Considering you can encounter unique monsters in Whimsyshire named Midnight Sparkle, Rainbow Western and Unique Nightmarity, it's pretty clear that bronies influenced them to some degree. At the very least, Blizzard saw the resemblance and decided to throw in a few references.
The Mole: It turns out that Adria has been serving Diablo since she first met Aidan, who had attempted to contain him within his own body, and had conceived Leah with him for the sole purpose of using the poor girl as a vessel for Diablo's return as the Prime Evil, in accordance with Diablo's grand plan.
Money Sink: The game has auction house fees and high repair costs for top-tier items, as well as a few scattered one-time costs: artisan training, storage space increases, and access to the gag level on higher difficulties. Crafting also serves this purpose, with the blacksmith taking the place of gambling in Diablo II and the jeweler upgrading gems which level to level have a linear power boost for an exponential cost increase.
Money Spider: You'll discover riches from spider eggs, cow corpses, angelic vases, ghosts, and bees.
Coven Member: He's/She's unstoppable! No mortal could slaughter our brethren with such ease!
A journal you find in Act II called "The Feared Hero" confirms that the remnants of Maghda's coven are scared shitless of you:
Dark Cultist: We camp, lying in wait for a hero of incredible prowess. My gut churns with the suspicion that we are simply fodder. I have heard tales of this hero wading through our ranks, slaughtering us as if we were children. I will not sleep again tonight, I fear.
Multishot: The Demon Hunter has a skill that's actually called Multishot. It shoots a large burst of multiple arrows in a conical spray in front of him/her, damaging any enemies in it.
Mummies at the Dinner Table: A random event in the Fields of Misery involves a farm besieged by Leapers. Once all the leapers have been killed, a man comes out of the farm cellar, telling you that him and his wife have been trapped down there by the leapers for hours, and that his wife would love to thank you for rescuing them. He leads you into the cellar, and introduces you to his wife...a skeleton sitting in a rocking chair. He says that she's been unwell of late, but his love will see her through! Then her head falls off. "Oh, she's nodded off."
The Gibbering Gemstone, one of the items necessary to open Whimsyshire, is a clear reference to the Chat Gem in Diablo 2's chat lobby. Its flavor text is as follows:
Chatty Gem You feel like talking to someone. It seems to be active, but it is difficult to tell.
Messerchmidt's Reaver has a description lampshading its popularity in the first game's final stages:
This mighty weapon once hewed its way through the demon-infested catacombs beneath the old Zakarum cathedral in Tristram. Could it be the weapon the warrior Aidan used to defeat Diablo all those years ago?
The description of the Stone of Jordan lampshades its status as common currency in the late levels in Diablo 2.
The Stone of Jordan is far more valuable than its appearance would suggest. Men have given much to possess it.
Adria is being held prisoner by Goz'turr the Torturer.
Certain combinations of monster pack Affixes, especially in Inferno, require a lot of effort to take down depending on your character. (for example, Vampiric+Molten+Plagued+Extra Health for melees or Arcane Enchanted+Vortex+Mortar+Frozen for the ranged) It's either you kill it or skip it.
Near Villain Victory: Act 4. Diablo and his demons have besieged heaven and are easily winning. All hope seems lost, and they seem destined to win... until the player comes along and defeats them.
Diablo is literally one step away from victory after you beat him in his Realm of Terror- he stomps the Crystal Arch and cracks it. If he had stomped another time, the Arch would have broken, the heavens would have be destroyed and he would have won.
Neck Lift: If you get caught in Diablo's Bone Prison attack during the final showdown, he will do this to you and drain your health before slamming you to the floor with authority.
Nephilim: "Nephalem" are the offspring of angels and demons, more powerful than their parents. Puny Humans are the Depowered version thanks to the Worldstone nerfing them. Your character is a "new" Nephalem that has emerged thanks to the Worldstone's destruction.
New Game+: Just like in the other Diablo titles, after finishing the game on one difficulty, you can access the other difficulties with your upgraded character.
Largely averted come patch 2.0. You can access nearly all difficulties from the start. Only Torment is locked until you have at least 1 character at max level.
The hero unintentionally helps bring Diablo's resurrection due to getting duped by Adria.
Tyrael's fall awakened the Skeleton King.
The Angels and Demons that created the world of Sanctuary. They were just renegade angels and demons that wanted peace. Yet their off-spring, the Nephalem (ancestors to the human race,) turned out to be so powerful that they were dangerous, which created all the conflicts in the whole game's history.
Baal's corruption of the Worldstone. Tyrael's destroying it led to the reawakening of humanity's nephalem power, which culminates in the PCs being able to defeat the last Lords of Hell and then Diablo.
Azmodan comes right out and shows Leah where he's going to invade, destroying his chance at a surprise attack and giving the hero the opportunity to stomp his ass. He then proceeds to announce every sneak attack and infiltration he tries in the actual battle, dooming each in turn.
Noble Male, Roguish Male: Kormac the Templar (noble) and Lyndon the Scoundrel (roguish), two of your three followers. Kormac is a noble paladin type who falls hard for Eirena, your third follower, while Lyndon is a quite unrepentant rogue who is not nearly as good at attracting women as he thinks. This being Diablo, the two have Hidden Depths — Kormac is revealed to have been lied to and brainwashed by his order, while Lyndon is doing everything in his power to pay off the Merchant's Guild and get his brother out of prison.
Noble Savage: The witch-doctor and the barbarian both fit this. The templar almost quotes the trope name in a bit of dialogue towards the end of the game.
No Honor Among Thieves: It's brought up as possibly the reason the forces of Evil have never managed to defeat the forces of Good. And why all the hosts of heaven can't stop Diablo when all of the Great Evils are contained within him.
Nonhuman Humanoid Hybrid: The ancestors of humans are half-angel half-demon hybrids called the Nephalem. It turns out your character is the first of the new Nephalem.
Non-Human Undead: The game gives us the Witch Doctor class, who can raise zombie dogs and the "Gargantua", who is made from an undead bear.
Noob Cave: The old Tristram Cathedral, the main dungeon from the very first game, which you take two trips through — one to rescue Diablo regular Deckard Cain, and the other to find and destroy the Skeleton King, which has been resurrected by the power of the Fallen Star that fell on said cathedral, and which you have been tasked with getting to the bottom of.
Scoundrel: Angry naked women are attacking me? This is my summer in Westmarch all over again!
Not So Different: Some characters, such as Zoltun Kulle, argue that the angels are no better than the demons, and both sides would destroy humanity if they felt like it. Even Deckard Cain fears that this may be the case.
Cain: Angels. Demons. I fear their conflict will soon engulf the world of men. And when it does, what hope do men have when even the wrath of angels cannot be quenched?
Oblivious to Love: Eirena the Enchantress seems to have no idea how Kormac the Templar feels about her. The player character has to make it clear for her.
Eirena: The Templar's so strange around me at times. Barbarian: He's in love with you. Eirena: What?! Barbarian: You never noticed? Eirena: No!
Obviously Evil: Zoltun Kulle. Intentionally non-spoilered due to how hideously obvious it is and how often Tyrael and the Templar point it out. It's not a case of if he'll betray you, it's when. Too bad the claims that suggest he's betraying you prove to have been sound advice.
Offing the Offspring: Adria, in the cruelest betrayal of the entire series, kills her own daughter Leah by shoving the Black Soulstone into her chest and using her as a vessel for Diablo's rebirth as the Prime Evil.
You encounter the ghost of Leoric's queen carrying around her own severed head by the hair. Later in the level, you get to watch a spectral reenactment of her execution by guillotine while Leoric and the Archbishop watch on.
The Demon Hunter can do this to their enemies if Bola Shot is selected and a killing blow is struck with it.
Malthael does this to two Horadrim in the cinematic trailer for Reaper of Souls
Oh Crap: During the Reaper of Souls epilogue, Tyrael has a subtle one, realizing that you have bested both the Demons' and Angels' greatest forces, and that as a mortal, even you are susceptible to corruption, hoping that you will be able to withstand the temptation should it present itself.
One-Hit Polykill: The Demon Hunter has several abilities that work like this. The Elemental Arrow, regardless of rune, is basically Area of Effect damage in a line, and some runes make that line narrow enough to resemble this. Some other shots can be modified by runes to hit multiple enemies in a straight line. However, it's unlikely to be a one hit polykill unless your character greatly outlevels the content or your targets are a very weak type of enemy.
One-Man Army: No matter which hero you choose, you will singlehandedly kill thousands of enemies over the course of the campaign. You and your companion get to save both Sanctuary and the High Heavens from Belial, Asmodean, and eventually Diablo himself! Lampshaded near the end of the first act, when the player character interrupts a coven meeting. The cultists can't believe that a single person is causing them so much trouble.
One Stat to Rule Them All: Every offensive skill scales from damage determinating your main needs. Vitality comes close to second to balance survivability.
Belial does this in the final Act 2 fight, transforming into a giant version of himself that takes up half of the screen.
Wizards can pull this off with their Archon ability. It turns them into a being of pure power, with extremely powerful melee attacks, powerful AOE abilities and a devastating, upgraded version of Disintegrate.
Dark Vessels, who can transform into Dark Thralls. However, the transformation ritual takes a long time to perform, they're helpless while they perform it, and until they do transform, they have such low health that a character with strong enough attack can kill them before they transform.
In Reaper of Souls, Adria transforms (or is transformed) into a monstrous demonic form with huge skeletal wings that are used to both walk and fly.
Ontological Inertia: At the start of the game, five of the seven Lords of Hell have been trapped within the Soulstone, with a sixth joining them at the end of Act II. Despite the most powerful of their number being absent, the Legions of Hell are just as credible of a threat to both Sanctuary and the High Heavens.
Our Angels Are Different: Diablo angels wear heavy armor, hoods, and have permanently shadowed faces. The only halo around is Imperius's, and it's made of steel. Their most striking feature are enormous flowing tentacle wings made out of glowing energy which can be used to manipulate objects as well as fly. Angels themselves are said to be made out of harmonic vibrations and light.
Many angels don't have a rosy view of humanity. Mankind itself was created by the union of a demon and an angel; one of them saw humans as slaves and worshippers and the other was the demon.
Outgrown Such Silly Superstitions: Toward the beginning of Act III, Tyrael sends for reinforcements from a nearby king. His response is that he will not send real men to fight imaginary demons.