The content will include four Halloween-themed acts, new paid PvP tournaments, jumping puzzles, mini-dungeons and Halloween-themed items.
'Shadow of the Mad King' is the first major expansion for to be released for Guild Wars 2 since the game went live in late August.
The update will focus on the possible return of Mad King Thorn - for most people, simply a myth and a "figure of folklore".
Guild Wars 2's launch was marred by reports of mass phishing attempts against its community over the Labour Day weekend.
It was claimed that more than 11,000 support tickets over faux emails designed to steal user data were logged during the three-day period, although NCSoft and ArenaNet have since downplayed the figure.
'Shadow of the Mad King' goes live on October 22 and the new acts run through October 31.
> Read Digital Spy's review of Guild Wars 2
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