Thinking about it, that statline is quite nice. 16W @ T7, enemies are at -1 to hit with no rerolls, ranged attacks are at -1 to wound, and he has a 4+ invul save. He's likely lost his rerollable saves (that was the old Shadow Form ability), but all those together are still quite good.
For context, these are the rules that have been revealed so far:
Now, one thing that I found interesting was that Immortal Vigor and Shadow Form are separate abilities. You would think that if these were going to be unique abilities for Be'lakor, GW would simply roll all of the defensive effects together into a single ability. I wonder if this means that we'll see Immortal Vigor on other units, like Greater Daemons? That would be a very welcome change.