I saw this on youtube today and had a check round the major forums and sites and no one is mentioning it. Not even a mention the Guild Wars 2 website, twitter or ArenaNets website.
From what I can tell this is either a promo video for the Ascalon Catacombs dungeon or the actual in-game cinematic you will see. I’m really hoping it is in-game and that these awesome concept art cinematic’s happen fairly often even outside of your personal story.
Check out the video for yourself: