The Dark Elves. The Dark World of the title is Svartalfheim, realm of the dark elves. So, pretty literal subtitle.
But what sealed it for me was a brief shot of a baddie with pale skin and half his face blackened and scarred. Not as bishy as I'd have made him, but it was pretty definitely Malekith.
ETA: Turns out this has been known for a while, the ads are just being unnecessarily coy about it. Almost everyone who would recognize the name of the baddies reads the spoiler sites anyway, so they could've named the baddies in the trailer without giving anything away. ;)