Hellhound is an interesting and pretty new ability of the vampires, but it is rarely used. I believe there is one main reason for this: The range of the ability is too short. It would be nice if you could cast it over cliffs, such as the one to the slayer pool. As it is now, the hound will simply spawn right next to your vampire. With buffed range, you could also spawn it inside someones base and try to snipe their hero. To compensate for this, you could make the hound melee instead of a ranged unit. But it would probably need to have a slightly longer duration if it needs to run up to the slayer as well.