I thought with no less then 3 new installment announced, it'd worth to investigate the franchise.

The gameplay is addictive, the games are usualy innovative for their time, they are full of bugs and design-failures, but what about the lore, I thought.

So if you have any question, feel free to add.
If you have answers, all the better.
Also, if you know what source-materials exist, that'd be even greater.

For sources, I have access to most [url=https://en.wikipedia.org/wiki/Diablo_(series)]listed here[/url], with the exception of Adria: a diablo beastiary (anyone knows why there is no ebook version?).
And of course Arreat Summit still exists too, and there were those short stories, D&D-campaigns... Whatever else I manage to bump into.

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QUESTION #1: the D1 manual tells Lazarus shattered Diablo's Soulstone. What? How? What did the Warrior stuck in his head then?
ANSWER: "shattered" likely means "got damaged", like Baal's soulstone, not actualy shattered like a glass vase.
SUBQUESTION: A slight problem still meains, namely that the manual says the Soulstone was planted in the chest of the kid, but in the cinematics it is removed from the forehead.

QUESTION #2: at one point, entering that hut in D1's intro, why did the Warrior remove his helmet (thenp put it back again)?

QUESTION #3: what's the significance of the sword shown in the D1 intro?
Possible Answer: it might have been intended to be Azurewrath, but Blizzard proving incompetent again in organizing dropped that idea after the cinematics was done.

QUESTION #4: Why Tyrael does not take responsibility for the Resurrection Spell? It was so obviously him.

QUESTION #5: if resurrection is only possible at the place of death, how is you resurrect your mercenary in the towns?

QUESTION #6 How did Baal's soulstone shatter, so Tal Rasha had to volunteer?

QUESTION #7: Decard Cain lies in Diablo 1?
So as the story goes, in D1 Prince Albrecht was taken to the labyrinth, were got possessed by Diablo, who made his nightmares of monsters come alive.
Then in addition there is the rumour that some people got dragged to the labyrinth - including Wirt, who got rescued by Griswold.
This is fine and dandy, but here is the problem: Decard Cain gossips about Wirt telling that the two boys (Albrecht and Wirt) were taken the same day! As no creatures existed until Albrecht was taken, this would be impossible!
Should have been a VERY busy day, as first Albrecht had to be captured, Leoric send out an investigation, order execution after the searching and interrogation failed, then Lachdanan return, kill the king, the people move out of town, cloacked figures accost people on the roads, rumors about these riders spread, AND monsters invade the town to capture Wirt! That's not working out for me.

QUESTION #8: How did Decard Cain memorize the Identify Spell?

QUESTION #9: What does Adria do in Tristram? If she is in cahoot with the evil, why she supplies you?

QUESTION #10: Prophecy about Wirt?

QUESTION #11: The manual mentiones some "cloaked riders" on the roads around Tristam, clearly the same faction as Ogden calls "dark riders" in his welcoming speech, the later supposed to have massacred Tristram itself.
I suspect, given in some interview Diablo was told to be an action-version of Warcraft, that these "riders" are leftover from the designer-board. My hunch is, they were initialy planned as the bad guys of the game, but the plot got altered by a lot, and they simply forgot about it, and for the manual they were altered as simply rumour, and not even in Tristram.

QUESTION #12: Who is the sacrifised child at Lazarus?
ANSWER: despite what is implied in the scene, it is NOT Prince Albrecht. Likely some other child who was dragged down, like Wirt.

QUESTION #13: Why we dpm't see Nlood Raven or The Summoner use their D1 skills (aside Summoner's firewall)? Where did they learn they D2 skills? The we don't see the Summoner summon something?

QUESTION #14: why are there only 2 type of scrolls in D2?

QUESTION #15: how did the Sabre Cats end up in Aranoch?

FOKUP #2: Westmarch is in the wast on the D2 Manual's map. Ok, it is on the north as it should be according to the D1 Manual if you turn the map to match the climates. But the problem is, on the D2 map east is opposite to Westmarch, while in D1 they were adiacent.

FOKUP #3: the D2 Manual mentions The Time Of Troubles, which means they left in a source of inspiration/copyright infringement of Baldur's Gate/AD&D. The period in the Diablo-universe should be named The Dark Exile.

QUESTION #16: Why the hero actualy planted the Soulstone in his/herself at the end of the game? Remember, that only the Horadrim and Tyrael knew how to use the soulstones, the hero did not!
ANSWER: The hero likely got corrupted already when reaching to Diablo. Given that none of the three heroes came with heroic intention, but were motivated by simple greed...

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Processed sources:
Diablo 1 manual
Diablo 1 ingame content
Diablo 2 manual

The timeline is generally inconsistent. Wirt gossips:
---
I don't sell information, and I don't care about some King that's been dead longer than I've been alive. If you need something to use against this King of the undead, then I can help you out...
---
Taken at face value, that would suggest that King Leoric was killed before Wirt was born, which contradicts parts of the timeline you gave above. The second part makes it pretty clear this was meant to refer to King Leoric specifically, not to Wirt being generally uninformed about who the recent kings were.

Someone (Ogden, I think) gossips:
---
Some months ago King Leoric's son, Prince Albrecht, was kidnapped. The King went into a rage and scoured the village for his missing child. With each passing day, Leoric seemed to slip deeper into madness.
...
The King's Knights and Priests tried to placate him, but he turned against them and sadly, they were forced to kill him.
---

So, (1) Wirt and Albrecht were taken on the same day, as you say. (2) King Leoric was killed before Wirt was born. (according to Wirt's gossip) (3) King Leoric was killed after his son was taken (according to Ogden's gossip), which was only months ago. From this, we can conclude that Wirt is surprisingly big, well connected, and well spoken for a child of less than a year old. :)

Those lines might be quest-related, not simple gossips which I'm currently reading through.
Will give some explanation if I find any, until that I take notice, and let me give a theory:
(1) is a lie, or somehow Decard C. is misinformed.
(2) can be exeggeration. Or simple neglectful behaviour. He might not even know who kingLeoric was, he possibly only catched it is a sekelton undead.
(3) dunno about Ogden (might be mid-quest-talk), but on the timeline I currently have, the taking of Albrecht/Wirt happened years before Diablo 1 happening. That's enough time for Wirt to make his empire, especialy that he is in a hurry to collect a large amount of sum.
*We can also hope, some of the inconsistencies will be removed by other sources, clearing which quests got canon, quests being Randomly Exclusive.
**From other dialogues it is clear Wirt only became involved in the shady business after he got dragged to the labyrinth. So the monthes ago being taken must be false information. But falso information from Decard Cain? DC being an untrustable narrator? What else he got wrong, or qwhy did he lie?

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Similar problem with the Butcher quest, where it is heavily implied, the storming of the labyrinth led by Lazarus happened like an hour ago or something.

Based on the amount of unfinished/cut content in the game, and oddities like that the Wounded Townsman can lay there "dying" indefinitely until he talks to you, I choose to believe that this is a result of a rushed release, where consistency checks were skipped. Perhaps different people wrote different dialog bits, and did not coordinate on the background described in that dialog.

QUESTION #17: why the mercenary does not resurrect you in D2 (hardcore)?
Answer: The D2 Manual tells the mercenary dies too if the player falls. They are that weaker on plot armor.

FOKUP #4: the tomes Wages of Sin are War, and Tale of the Horadrim both claims to be the last legacy of the Horadrim

FOKUP #5: Tyrael in the Act 2 cinematics says "Hell" instead of "Hells".

QUESTION #18: Why Tyrael didn't prevent Marius freeing Baal?

QUESTION #19: How did Tyrael know the Gates of Hells had opened in Kurast? Why didn't tell it to anyone? Why didn't do anything about it?

QUESTION #20: How did Mephisto escape from his soulstone?

QUESTION #21: Those who talk about the Wanderer in D2 why none mentiones Marius?

QUESTION #22: why waste time bothering with the Tree if Inifuss?

QUESTION #23: why was Cain not killed in Tristram?

QUESTION 24: did Diablo have a Horadric Staff (both rod and headpiece) and a Horadric Cube to get to Baal?

QUESTION #25: Decard C. says this: "be prepared for a fight - you'll likely have to kill Tal Rasha to destroy Baal." - but if this was always an option, why didn't they do it?

QUESTION #26: Isn't it a bit convenient the Claw Vipers just came up with their sun-darkening ritual, without you'd have like never found a horadric staff headpiece?

SOMEWHAT OF A QUESTION: How exactly did Diablo know where The True Tomb Of Tal Rasha is?
I suspect the Summoner had his hands in this.

QUESTION #27: Where is Marius while the hero travels to Kurast? He had to travel on Meshif's boat the same time I feel. How did he arrive there first?

QUESTION #28: What's the problem of Tyrael in Act 2 of D2? Why this "need to know" information giving? Why no mention of Marius' journey?

NOTE: I find it interresting that D1 and D2's lore are almost entirely distinct.

QUESTION #29: Why is the D2 protagonist called "avenger" at (not class specific) greetings (I think in Act 2, and also by Tyrael in Act4)?

QUESTION #30: How does Tyrael know Mephisto is defeated?

QUESTION #31: Cain is either moronic, or otherwise entirely untrustable. He says, I quote: "Diablo still roams free in Hell, marshalling his demonic forces. Only when he is beaten will our world finally have peace." Did he like forgot about Baal? What is Decard Cain's purpose? What is the deal between the Horadrim and the angels - or mor precisly Tyrael? Does Tyrael work with the angels, or he has other plans?

QUESTION #32: What happened to the Azurewrath?

QUESTION #33: Izual.
My problem with Izual is many. I could start with why the body-swap? Or why is he still in Hells is he's not trusted by the demons, why not banished into a pocket-plane, or eliminated? We KNOW it is possible, as we KNOW at the end of the quest he is "sent to oblivion", aka. is destroyed for good. Or why did Tyrael let him out of sight, especialy on that suicide-mission if he knew about the soulstones? Or even, why did Tryael not just tell him about the Soulstones, but also about they stones' weakness/corruptableness?
Sure, this moment puts a twist on the scale on the demons' side how big the plan is - but why stop here?
The most interresting bit of Izual's monologue for me is this part: "The plan we set in motion so long ago cannot be stopped by any mortal agency."
This not just repeates the same information. This tells, that Izaul came up with the Dark Exile's idea in the first place! Izual is not some turtured, unfortunate soul who broke under interrogation. Izual did not go to destroy Shadowfang. Izual did not plan to return.
You see it was not the Three's plan. It was "our" plan, suggesting Izual volunteraly went there and gave up the information.
But who is Izual? He is a throwaway character, has no significance, no screentime. Surely then he wasn't the mastermind behind ... like anything. I suspect, this story tells me, behind everything stands the enigmatic archangel, Tyrael, who gave the Soulstones to the mortals/humans in spite of Heaven's will. Because it was his plan.

QUESTION #34: it is told by Tyrael, and D2 obviously handles as such, that destroying a Soulstones destroys the one of the Three who is bound to it.
But Tyrael also says this: "Diablo and Mephisto have been banished back into the Black Abyss that spawned them".
So, the question is, which time did he lie?

QUESTION #35: Is Natalya one of the seven protagonists?

QUESTION #36: How is anyone in the Guardian Tower if you can enter only by destroying the Compelling Orb, and that's only possible by Khalim's Flail...
ANSWER: the orb was made by the corrupted Zakarum members.

QUESTION #37 How could Mephisto do anything from inside a proper Soulstone?

QUESTION #38: this Khalim-business must be a pretty recent development, as things were surly fine at least untiol Meshif's last visit. So Decard Cain should not know about all this, since he was in Tritram, then journied through D2. What is going on? Is DC just holding us up?

QUESTION #39: Come to think of it, how did Marius get inside the Blackened Temple?

QUESTION #40: Why build a Tower to place the beast below ground? - good question, Hratli. What was/is the REAL purpose of the Horadrim - or should I say Tyrael?

QUESTION #41: We have seen Diablo, but remain unsure of his Brothers' whereabouts. - good question, Alkor.

FOKUP #6: The D2LoD Manual marks Qual-Kehk as an elder. That's not true in the actual game.

QUESTION #42: Malah, the healer of Harrogath, can not resurrect. Who is resurrecting the mercenaries then?

QUESTION #43: how do we know the Worldstone got corrupted, thus had to be destroyed?
Actualy, coming to it, what IS the Worldstone?

QUESTION 44: If Tal Rasha was a Horadrim INITIATE as the D2 Manual says, who could he be "one of the most powerful of the ancient Horadrim" as Cain says? Or Cain again spreads false information? He seem to have a tendency to do that.

NOTE: Another factor to rise suspicion towards Tyrael was the introduction of forces believing/following neither Darkness or Light, demons or angels.

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Processed sources:

D2 LoD Manual

D2LoD ingame content.

Sword of Justice (comic)

Tales of Sanctuary (comic)

FOKUP #6: in the majual, the necro's god is always spelled Trag'Oul, while in-game and on the Arreat Summit it is Trang'Oul.

QUESTION #45: where did the Sand Raiders come from, them being from another dimension?

QUESTION #46: how can 4 spieces of putrid defiler create 5 species of pain worms?

FOKUP #7/QUESTION #47: Arreat Summit says this: "Over the next two centuries, the three imprisoned brothers focused their Demonic powers upon corrupting the Soulstones that bound them to the mortal plane and began bending the will of any nearby humans. Mephisto was the first imprisoned and, consequently, was the first to break free from his bondage."
Adding that there is no reason given how Mephisto got to his influence, can we say the very point of the sacrifice of Tal Rasha just became moot by retcon?

QUESTION #48/FOKUP 8: Adding to the above, the Arreat Summit tells this too: "As punishment for his part in the Dark Exile that sent the Prime Evils to the mortal realm, Duriel has been left behind by Baal to guard the Tomb of Tal Rasha. Embittered by this new position of solitary servitude, the Lord of Pain has claimed the souls and blood of many bold adventurers. "
The problem is, how far the protagonist of D2 was behind The Dark Wanderer? Hours? How many adventurers could reach the LOST TOMB OF TAL RASHA, ONLY UNLOCKABLE BY A COMPLETE HORADRIC STAFF in that time?

FOKUP #9: In Sword of Justice Mount Arreat got moved compared to the D2/Lod Manual's map for a better narrative position.

QUESTION #49/FOKUP #10: How does Bartuc appear in LoD? For D1 we can assume that quest didn't happen, but LoD is a different issue. The problem is, Bartuc was deemed dead since his confrontation with Horazon.

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Processed sources:

Arreat Summit

The Secret Cow Level D&D campaign

D2: The Awakening D&D campaign (clearly an alternate universe source, where claw vipers are under Tristram, Barbarians can't specialize into weapons, necromancers cast hydra, and rogues are confused with amazons on the creators' part amongst others)

D&D Adventure Game D2 edition fast play game (I do not have access to the full version - which simply lacks the fast play game subtitle -, but seems to not touch the existing lore)

D&D: D2: Diablerie + D&D D2: To Hell and Back are sister-book, the previous being the player's handbook, the later the DM's. Pretty good conversion, but Diablerie contains no lore, and HaB takes its stuff pretty much directly from the game, with the exception that the angel at the reiver is just a projection.

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I was running through Diablo 3's lore, and had to realize, any idea the Diablo-lore had, got squandered, tossed into the garbage-bin. If you trim down the stuff that has no effect on the story, then you are left with D1-D2 stuff, no Sin War, no Dark Exile, no Soulstones even, just "Diablo&Co comes to Not-Earth, start to wreak havoc, player kicks their ass, there was no plan, now go home"..

So despite hunting down every single lore-material, I will just delete them from my harddrive.
But befre that, I give you the chronology of the games, novels D&D-modules, comics.
Ok, I don't realy have idea where the short stories that didn't end up in Heroes rise... fit, or where Morbed, Book of Cain/Tryael/Adria fit (Mordred mybe before D3, the rest during D3 probably).

chronology:
https://ibb.co/g9s7X4d

Regarding all questinos about intro: ignore it. This cinematic is based on outdated scenario for the game, that was changed to what we have now during development. But since making cinematics was a hard work, they just put what they had.