My theory is that the 5th race is either the Darkness itself or a race completely subservient to the Darkness. The concept looks kind of like a satanic priest so my guess is that they worship the Darkness. I don't think we will fight them till the very end kind of like with the Flood in Halo.

Bungie released some concept art last year that showed 5 races. 1-4 are the Vex, Fallen, Cabal, and the Hive, and haven't said anything about the 5th race. You can see the concept art here. http://static3.wikia.nocookie.net/__cb20130329155312/destinypedia/images/7/73/Races.png

#Community. Every Thursday afternoon. Watch for a post from DeeJ

Follow the [url=http://www.bungie.net/7_Bungie-Mail-Sack/en/Groups/Detail?groupId=39699]Bungie Mail Sack group.[/url] DeeJ usually puts up the question thread on Thursday.