I believe that we will be fighting one of the Iron Lords as the final Raid boss. I know it sounds crazy but I have plenty of lore/speculation to back it up so just stick with me here for a sec. In numerous Iron Banner related artifacts for various classes, it references that the Iron Lords had a falling out. The one I'll pick from mainly is the "Head of Seogoth". It contains a quote from Gheleon reading, "I thought we formed the Iron Banner to fight the darkness, not ourselves. Doesn't sit well with me s'all I'm saying." Now you could assume this means the Iron Banner Crucible event, but Gheleon died before the Iron Banner event was put in place by Salidin. I believe the Iron Lords found SIVA and disagreed on what they should do with it. Remember, SIVA is not inherently destructive in function. It is just a technological substance used to transform biological and nonbiological matter into technology compatible with SIVA. I believe that one of the Iron Lords wanted to take control of the Darkness with SIVA and make it into an army. One thing led to another and he attacked the rest of the Iron Lords. They were killed and he was sealed inside of the Warmind bunker. Salidin was the youngest of the Iron Lords, having only joined their ranks after the battle of the Twilight Gap. He was most likely the one left behind to seal them inside, to stop the Corrupted Lord. Now centuries have passed and the Corrupted Lord has most likely become completely infused into SIVA technology. Then the Fallen find him and he begins his plans to create an army and take revenge on the Last City and Salidin. But that's just a theory, a game theory.