The instance is called Maw of Souls, and was added recently to the forthcoming expansion's alpha build. It's one of the nine five-man dungeons coming with Legion and it looks really impressive! The setting is Vyrkul-themed (basically it's Norse mythology) with appropriate misty, rocky shores and shipwrecks. Players are transported into Vyrkul hell where they have to cross the Sea of the Damned, riding on a ghost ship called Naglfar. So it's one of those instances where things are a bit more interactive (think Grimrail Depot from Draenor) and it's not just some static cave. We're on a ship in the middle of a giant storm.
The dungeon's short, the bosses look really fun, so hopefully the novelty won't wear off fast. The alpha version debuted yesterday and some YouTubers raced through it already. Here's Bellular's video:
And here's one from FatbossTV. Both clips show the bosses, the tactics against them and the whole environment. The final boss fight looks awesome!