Don't believe anything you hear about him. There is so much speculation being thrown around that people are taking as fact for some reason.
There war a shilouette picture of him from the early days of AoS where you would see that he's got wings and horns, so some retard stated it as fact that "hE hAs MeRgEd WiTh HiS dRaGoN!!!!" and a ton of people believed it for some reason.
The chaos god thing is because there was a wheel with chaos symbols where one of them looked like the iron-circlet that he wears.
All we know is that he mostly rules Ulgu, and has his own brand of "shadow-demon elves" or some shit.
Also he's a part of Sigmars pantheon, and is famous for making the sigmarines a magical colosseum.