Spoilers in the video, obviously. It's the first mission from The Taken King, called The Coming War.

YouTube Link.

I actually didn't watch all of it yet, so I can't really comment on content. Phobos is beautiful, though. Bungie skybox=best skybox.

EDIT: Just watched it. Looks fantastic. The Taken Psions look like they could potentially get out of control very quickly, which I think is great.

Also, not sure what it was exactly, but the level and encounter designs looked the most Halo-like we've seen from Destiny so far, which is a great direction to be going, as far as those things are concerned.

All in all, if the whole of TTK meets or exceeds what we just saw there, it's going to be great.