it was not a faceless, as faceless already had an in game model, and it didnt look like a faceless at all.
it looked like C'Thun, how ever i doubt it was an actual old god.
it couldnt have been Yogg'saron could it?
my own suggestion is, that its a minion of the old gods like the faceless, just a variant we havent seen. and the fact we didnt see this variant in Northrend, saddens me somewhat.
edit: it should probably be a minion of Yogg'saron since.. well that made most sense at least to me.