“Durial321 is a banned RuneScape player and a bug abuser.”
RuneScape is a popular MMORPG that reached its peak popularity in the late 2000s.

In the game, combat – colloquially known as PvP, or player vs. player – is limited to a specific map area (called the Wilderness) and otherwise people’s houses.
On 6/6/6 (sic!) a bug in RuneScape made it possible for a few players to start killing others outside of designated areas, without them being able to defend themselves. One of these players, Durial321, gained a lot of notoriety:
A player called Cursed You had invited some friends to his in-game house once he had maxed his construction skill, but decided to eject them all from the premises. Things turned sour, however, as a group of players marked as PvP in the house didn’t lose this PvP flag when ejected, allowing them to storm through Falador and massacre whoever they pleased. The most notorious of these players was named Durial321.
This event went down in internet infamy and meant that many players lost their items when killed as well as the banning of those involved.
I don’t have any context of RuneScape and I found it really funny to learn about this event from different retellings of the story.
This wiki entry reads almost as journalism:
Several others were able to use this glitch, but Durial321 abused it the most. His rampage lasted for about an hour, starting at Rimmington, where the house party was, then proceeding to Falador and subsequently Edgeville. At Edgeville, he gave Voodoolegion the green partyhat, who never gave it back to him. Soon after, he finally encountered a Jagex Moderator, Mod Murdoch, who disconnected him and locked his account. Durial321 was later permanently banned from RuneScape. In a 2006 interview, he said that player killing outside of the Wilderness was exciting, although he felt bad for the players who lost their belongings.
The 2006 incident later became known as the Falador Massacre.
(The tone is even more funny if you actually read the interview.)
There is also this more modern retelling that feels like scary story time by the campfire:
Reactions from players were initially kind of incredulous. Plenty of people were shocked and found the whole incident quite funny. Durial had essentially broken the game, after all. Some players wanted to be like him, whipping strangers to death and taking their items. But soon, as more players started hearing about what had happened and seeing the video, the mood shifted. Players wanted Durial321 hung, drawn and quartered, with his head displayed on a pike outside Lumbridge Castle.
You can witness the event PC Gamer called “one of the best all-time MMO bugs” by yourself since there is video capture of the Falador Massacre taken by one of the witnesses. At least to me, however, it’s rather incomprehensible.
Fear not, however, because there are many (!) documentaries. This recent one is reportedly the best one and also goes into the technical details:

Without spoiling too much, the bug was a classic Swiss cheese situation involving a new untested item, a race condition, peculiar timing, and a player with an unusually high uptime and a whole lotta luck.