That's Sandal's hideout.
Seriously though, that's for a DLC.
But... more seriously I'd say that it's just an obviously-incomplete portion of the region that had been planned. At some point the devs went: "Crap, we're out of time, need to do something about that part of the map over there, suggestions?", and one guy from the level design department went: "Hmmm, what about covering up the entrance with some debris and a mountain face?". And so its entrance became "hidden". The level designers then probably went like: "Ok... hopefully no one is going to jump on this rock and think about walking forward that way...". And upon seeing this video the same guy(s) went: "Damn, told you we should have actually blocked that rock! Now they'll keep saying it was removed content for a DLC!".
And that's right, BioWare, that's right. We've discovered your secret. Now just announce the DLC, you have your back against the wall * cue in the dark instrumentation *