Developer poncle has confirmed that Vampire Survivors much anticipated online co-op update will fully release before the end of the year.
On their official Bluesky account they said "Online co-op is coming to Vampire Survivors on PC and consoles THIS Autumn! (yeah it's autumn right now shh)" and with Autumn ending December 21st - it won't be too long to wait. And on PC you can already try it out in the opt-in Beta on Steam as covered on GamingOnLinux at the end of August.
They're also teasing more announcements of extra features are to come and said to stay tuned for next week. They also gave the online mode its own trailer, which makes a joke of the "add online button" some developers make about how some gamers expect adding online play to be easy.

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