Continuity
For information on the topic of continuity in fiction, continuity errors, and retroactive continuity (retcon), see wikipedia - Continuity (fiction)
Basic back story of AC
The basic back story of AC involves the events of the Olthoi invasion of Auberean, Asheron's involvement, The Sundering, and the conditions which summoned Isparians to Dereth. This story has been told many times, and there is often inconsistency with these tales.
Versions
Original Intro Video
Asheron and a group of Empyrean mages are barricaded in a great hall - the mages sitting in a circle, Asheron behind them. Across the room from Asheron, the barricaded door is being pounded on, and just before it is breached, Asheron casts a powerful portal magic spell sending the other mages away. Moments later, Olthoi pour into the room and the ceiling begins to collapse. It then cuts to an exterior shot of the castle, which is now crumbling apart. It returns inside, where Asheron's hand is seen reaching out of the rubble.
At an undetermined time later, Elysa enters a portal to Dereth, with an embroidered picture implying that other species were also summoned at or around the same time. Elysa is Enslaved, but later escapes, meets Thorsten, ventures to Asheron's Tower, and meets Asheron himself. Asheron gives her a special potion that is deadly to Olthoi. Later, Elysa, Thorsten, and the other rebels assault the central hive of the Olthoi. Thorsten is killed by the Queen, and Elysa slays her afterwards. Finally, other Isparians are shown arriving at the now settled human outposts of Dereth.
A Stormwaltz quote states that the scene at the castle takes place on Dereth, and the mages with Asheron are adepts of the Knorr Lyceum. The adepts were the last Empyreans to leave. They had stayed behind to help Asheron evacuate everyone else.
Original Manual
Asheron accidentally opened the portal to the Olthoi world that set them loose upon Auberean. When their empire was lost Asheron cast the Sundering, and stayed behind to try to find a way to defeat the Olthoi. The Sundering directly and instantly caused portals to open on other worlds, including Ispar. Asheron saw great potential in the humans and began to concentrate on summoning more to Dereth. Many that arrived were enslaved by the Olthoi, but eventually, aided by Asheron, the Isparians won their freedom.
Historical Archive/Sybex Strategy Guide
The two tales told in these pieces are nearly identical, with just some minor wording changes in the strategy guide. This version is very similar to the one found in the original manual, however, it is more detailed, and it does not specifically blame Asheron for unleashing the Olthoi. Rather, it states that it was a magical experiment gone awry. The bulk of the text details early Isparians and the Olthoi rebellion.
A Time of Chaos Retcon
During the Olthoi arc, the way to the distant Knorr Lyceum was discovered. Inside, many texts written by Asheron were discovered. The story of the Olthoi invasion was changed, or further clarified, by these texts. It was not a magical experiment or accident on Asheron's part that caused the invasion - and his discovery of portal magic is only partially to blame, in that it enabled the exploration of other worlds. Instead, it was Gaerlan who inadvertently left a portal link to the Olthoi homeworld open. And it was Emperor Kellin who demanded that more Olthoi specimen be brought to Knorr, including the Olthoi Queen. With the Queen on Auberean, the lesser Olthoi began to obey her will again, and this is what caused Knorr to fall, and eventually, all of Auberean.
The manual included in Throne of Destiny has slightly revised text from previous manuals which puts the blame on Gaerlan.
The Knorr Quiddity Seed
This quest involves entering dream realms and defending Asheron's memories from Aerbax. It is an in-game retelling of the first scene of the original intro video, with Asheron and his disciples. However, this retelling has many errors. First, it takes place at the Knorr Lyceum. However, the events in the video are said to take place somewhere on Dereth, as it was the final stronghold of the Yalain people, where Knorr would be the heart of the Olthoi's territory. Second, in the video, the building crumbles to pieces, yet it is in perfect condition when we visit it in the present day.
Conclusion
Olthoi Invasion - The Olthoi arc retcon is considered the correct lore. Asheron was not directly responsible for the Olthoi invasion, it was Gaerlan and the Emperor.
Sundering - The truth about The Sundering remains a mystery. In all accounts, it did cause a disturbance in the fabric of Portal Space. But whether or not the Sundering caused portals to begin to appear on other worlds is up for debate.
The current Yalain Age is Moriqui Eipoth (ME), and was declared by Asheron after he sent his people away. According to information in the Ages of the Empyrean lore document included on the AC:DM CD, the date that Isparians overthrew the Olthoi was ME 579. This would put the date of Isparians first arriving on Dereth at ME 569. So the notion that The Sundering instantly caused portals to appear on Ispar is not correct, it took 569 years.
In the same document, it states that the Virindi arrived on Dereth before any other race, that the fought Asheron over something and lost. The fact that the Virindi are explicitly stated to be the first to arrive could imply that they tampered with portal space in some way that caused portals to appear on other worlds. Or perhaps this fight weakened Asheron, and some protection he placed on the currents of portal space fell. Whatever the case, it seems unlikely that the Sundering is the true cause, but the truth may never be revealed.
Undead
In early days, the back story of the existence of undead was very different from what it is now. While today undead are almost always members of the ancient Empyrean cultures of the Dericost and Falatacot, that was not always the case. The Zone Bestiary Undead Entry originally described them much differently. Undead were said to be the reanimated corpses of fallen Isparians, possessed by evil spirits said to be linked to Bael'Zharon.
During the first story arc this was retconned to the current Dericost origin of Undead. However, there are many early dungeons and points of interest that still suggest the undead have an Isparian/Shadow source. For example, the lore of the Fycheck Ruins near Shoushi state that the undead there are fallen Sho warriors who fled from battle. Another example is the somewhat common pairing of Shadow and Undead in early dungeons.
Tanada House of Earth
The Tanada House of Fire quest involves recovering special scrolls that belonged to the houses of Air, Earth, and Water. Tanada Sajo explains that "because Aerbax succeeded in corrupting the Masters of Air, Earth, and Water, he was able to steal the scroll of each House." However, the lore of the Tanada House of Earth quest implies that the House of Earth is unaffected by the recent corruption of the House of Air.