This guy should be the final boss of Legion. We have the models of all known Titans, except Sargeras.
So, is he/it Sargeras? No, he's not. Sargeras is probably doing well eons away of Argus. And one would think that if this model belonged to Sargeras, it will be more Legionnish (green fire and stuff). The fact is that he doesn't look at all like any picture that we have seen of Sargeras, but he certainly looks like a Titan, so, what the hell is this guy?
My thoughts on the matter are the following: Sargeras is scared of the Void, probably in his travels across the universe, he has seen a bigger manifestation of the forces of The Void, but he knows that he cannot trust any Titan (if they are more than the ones that he killed) or Baby Titans (world-souls are dangerous so they must be destroyed before The Void could corrupt them).
So he has a problem, he couldn't fight The Void everywhere, and his Legion is proving insufficient to help him in this matter. So he grab the souls of the Titans that he killed, and twist them, creating very powerful allies, ready to follow his commands, but far beyond his power, just in case something goes wrong. This is what he's doing in Argus, but we will avoid it.
BUT, not only he is trying to make the Titans his pets, he is experimenting with the concept of World-Souls. He's Sargeras, there is nothing that he couldn't achieve. So he tries to create the definitive weapon, Argus the Unmaker, a World-Soul created by himself, a very powerful ally. And he try to create this thing in Argus, a powerful planet, with the might of the Eredar to defend it, and with their intelligence to help in his development. And here is when it comes to mind, and makes sense, Kil' Jaeden famous quote: 'I paid for that destiny with my world'.
And how do you create a World-Soul by yourself? Well, feeding it with souls, a thing that the Legion collects by millions.