The "initial wing" of the first Guild Wars 2: Heart of Thorns raid is up and running. It's a ten-player adventure called Spirit Vale that has you chasing a missing Pact squad in the north of the new jungle area of Maguuma. There will be "dark secrets" and "strange magic" and, if I had to guess, an enormous tree monster boss or two (the concept art above does offer a clue).
It's a chance for Guild Wars 2 players to judge Arenanet's new squad UI, and there are legendary rewards to be won, eventually. Successful raids will "start you down the path to gathering the components you'll need to build legendary armour". Legendary items are hard to get in Guild Wars 2. The expansion has reshaped progression to incorporate top-tier guild crafting into the Mastery system. The Scholar of Secrets mastery is needed before you can start working towards the new legendary axe, pistol and staff.
A big patch accompanies the release of the raid which details the properties of the new enhanced squad UI, as well as bug and balance fixes. You can find the patch notes on the Guild Wars 2 forums.
My Asura necromancer is too busy getting crushed by the denizens of the Tangled Depths to even consider the raid just yet. It's a good expansion, though, as Phil noted in our Heart of Thorns review.