One of the things that puts me off that big unit is the potential for something like a dreadnought, imperial knight, wraithlord or wraithknight to charge it and make the dogs useless. Is there any way of making a khorne herald a threat to AV12 or T8? If so, that would be a high priority for me so while the dogs are getting killed you could be doing some damage with the character.
Ran into this issue against a Dreadknight. The Herald had a Greater Etherblade, so S6 AP2 Mastercrafted (S7 on the assault, but he didn't get it, what with the DK counter-assaulting after I wiped his Gk Termis and Librarian Warlord). That's enough to wound Riptides or Dreadknights on 4+, and glance AV12 on 6s (and there are just plain enough Hounds around that baring a disastrous boxcars Daemonic Instability test, no non-super-heavy walker or MC* will have enough attacks to wade through them all in a game). Against a Wraithknight, I think he might be better going for the Axe of Khorne on the Lesser Reward he has there, since S5 still wounds T8 on 6s--and the Axe of Khorne causes ID on 6s to wound.
The Imperial Knight... well, that's a bit more of a problem. You're still going to keep it absorbed for quite a few turns (again, barring boxcars--and probably all game if Be'lakor can spare an Invisibility for the challenge every now and then), since it's D-strength weapons can only kill 1 Hound per attack and its Stomp can't ID the Hounds. Still, it is a unit that costs slightly more than that Knight, and has no real way of touching it. Or Soul Grinders, or Ironclads. And they can't "our weapons are useless!" out, either. Your best bet would probably be to jump somebody like Be'lakor, Screamers, or Skarbrand (you know, the Armorbane brigade) in there to pump some pens into the AV13 walker.
*Barring something like Skarbrand with his 7 to 9 IDing Fleshbane attacks per phase, or something. There are always fringe cases. But I don't have to make that exception even for the anti-light-infantry-specialists the blenderfist Furiosos, I should think, given that I have a decent invuln, my Hounds aren't getting IDed by S6, and the Furioso needs to roll to hit on every new attack...