The One Sin (ザワン・シン) was known as The World's "unbeatable" event. Many players have tried and failed to complete it. It was finally conquered by Balmung and Orca, who would be later hailed as the Descendants of Fianna.
The Field
Located in Ω Sickened Imprisoned Fallen Angel, the field was unique. It lacked a map and music, and was covered with a blinding snowstorm. This combination meant that players trying to challenge the field would be easily ambushed by the field's high-level monsters. In the center of the field was a mountain of ice with stairs carved into one side. At the peak was the eponymous fiercest boss, The One Sin.
The One Sin
The One Sin (often described as a "Spectrum Dragon") seemed to take absolutely no damage while it dealt lethal damage with every blow. On top of that, its HP totaled over 65,000. The only way to cause damage to it is while it is using an elemental attack, and even then, only an attack of the opposing element can damage it.
The One Sin is a boss that counters every attack by using the same element used against it. Because the average player's first instinct is to attack, few people would actually see its pattern of attack.
Reward
The destruction of The One Sin meant the release of a captive angel (hence the field's keywords). The field would then become a normal field with no snowstorm or ice mountain.
According to .Hack//AI Buster, Balmung was the one who delivered the final blow along with doing the most damage, making him the MVP and having the Wings given straight to him, not Orca giving them to him
Revival
The One Sin quest appears once again in The World R:2. This time the monster appears in Morley Barrow, though, being a Lost Ground, this area may bear some connection to the field in The World.
Trivia
- The field of the One Sin event, Ω Sickened Imprisoned Fallen Angel, is a special field in .hack//Quarantine. Aside from Data Bugs being found within it, the Gott Statue at the bottom of the dungeon has the one-of-a-kind item "Risky Coffee" which restores HP and SP to full.
- The One Sin was improperly romanized as "Zawan Shin" in Tokyopop's first edition of Legend of the Twilight Volume 3. Although it technically is a correct romanization, the form has no meaning. This error was fixed in later printings of the book.
- In the Japanese version of AI Buster, two distinct Japanese meanings for the English are given: "The One Sin", and "The One Thing", playing on the ambiguity of the sound. According to "Tell Me Mr. Piroshi!", both are legitimate meanings of the name.