The final named chaos daemon lord is Cor'bax Utterblight. Fateweaver, and all the other named daemons in 40k don't get so much as a mention in this army, which is a pity, but I get the focus on the Ruinstorm and the daemons who played a much more prominent part in it. After all, there is a plethora of them. Utterblight is noted as a daemon prince in the background, and was summoned directly by the Word Bearers. However, there's precious little more than this, as one suspects that few survived the indulgences that this daemon unleashed.
As well as a very powerful stat line (Primarch levels, except for the BS), Utterblight is a Psyker (level 2).
Thanks to his big mouth, he gains instant death on a to-hit roll of 5 or 6. This is nice, and would not be much to mention except for the lack of eternal warrior in 30k (which Utterblight also has). He also causes d3 hammer of wrath attacks on the charge, and will "explode" upon his death to take some pyrrhic revenge.
The emanations of horror that he has is nothing too special, but does help once he gets in combat.
The fixed warlord trait, Pestilent Cloud, is not much to write home about. There's a lot better Nurgle powers than this.
The limitation of choosing the psychic powers from the biomancy discipline is a bit of a restriction. That said, in the hands of Utterblight, it helps him achieve his goals ultimately.
As with other Nurgle daemons, he is fundamentally slow. He needs to make the best of any combat that he manages to get in and use his psychic powers to maximum effect,
With a price tag like a Primarch, Utterblight at first sight seems very attractive. However, he has the same problem that most Nurgle daemons have: he is slow, and lacks significant ranged threat. In close combat: sure, he will be amazing. But overall, any White Scars player will simply run rings around him and diffuse his threat readily. Indeed, I would expect most space marine players to not feel too threatened by him unless they deliberately get in to close combat with him. Its a situation that an enemy can, in many cases, avoid. And therein is the fundamental limitation. He is a close combat specialist without being quick on his feet and lacks ranged threat. Angron does it better if I'm honest. Overall, I'm not really sold. Get a different HQ unless you're aiming for a truly fluffy Nurgle army.