From Diablo Wiki
Metcon is a portmanteaux combining "retcon" and "Metzen." It was coined by Flux of Diablo.IncGamers.com and refers to Blizzard's habit of rewriting the stories and world lore of their previous games to match their new or future ideas.
- Retcon: Retroactive continuity, or retcon for short, is the alteration of previously established facts in the continuity of a fictional work.
- Metzen is Chris Metzen, Vice President of Creative Development at Blizzard Entertainment and one of the key architects of their fictional worlds.
A "Metcon" or the act of "metconning" is simply a retcon performed by one of Blizzard's intellectual properties. Chris Metzen is not the author of all metcons, but the name works nicely and Metzen's status as one of the best known and most outspoken designers at Blizzard make it a fun term to use.
The term is often deployed sarcastically, but is not necessarily meant as an insult or a complaint. Many metcons have actually been improvements over the original ideas, in the opinions of most fans.
Metcons in Diablo
A prime example of a Metcon in Diablo 3 concerns the identity of the Dark Wanderer and the story of Diablo I.
In the first game there were three heroes, the, the , and the . All three had generic backgrounds and all were archetypal members of their classes without any real personality or individual character traits.
In the official game fiction/lore though, as advanced in Diablo 2, the Warrior was the one hero who "officially" defeated Diablo and took the soulstone into himself, in an ultimately-doomed effort to control the evil of Diablo's essence. The Dark Wanderer left Tristram soon after the end of Diablo I and traveled east to awaken Diablo's brothers Baal and Mephisto, a dark quest that formed the plot and story of Diablo 2.
This tale was metconned in Diablo 3, in fairly clever fashion. Rather than simply a nameless warrior, the character became King Leoric's older son Aiden, a character who did not exist in the original game. (Prince Albrecht was the unlucky youth was was driven mad and ultimately possessed by Diablo's essence, mutating him into the huge, red-scale and horned demon.)
Thus did Aiden return to his father's kingdom to deal with the aftermath of his madness, and to avenge his father, defeat his reanimated bones, and then kill his younger brother in order to break Diablo's possession of the younger prince. Furthermore, Aiden impregnated the witch Adria, fathering Leah, who went on to become the host of Diablo in his next human possession, during the events of Diablo 3.
Most fans considered that rather a good story and an improvement over the fairly bare-bones version of the story in Diablo I. It would arguably not have fit into the game at the time -- if the Warrior has such a deep relationship with the core events then it would have felt anticlimactic to have the Sorcerer and Rogue just be anonymous adventurers, and the Rogue obviously could not have have fathered Leah upon Adria -- but it was a pretty good twist to Metcon in years after the fact.
Changing Adria from merely an NPC to a scheming villain with a two-decade plan for demonic domination was also something of a change, but as that was just an expansion to the backstory, rather than altering anything established in the earlier games, it's not technically a Metcon.