We're told that specifically by the teaser page, that is.
So what will The Lost Shores actually entail? There's no way to know just yet, although some guesses can be made from the handful of teaser screenshots posted to the site, hinting at something vast being dredged up from the depths. What is certain is that this will be a one-time event, something that you can take part in when it happens or miss out on forever. Keep your eyes peeled for more information, and for now, just join us in wondering what might be down there a-stirring.
[Thanks to Prantik for the tip!]