Heart hounds were about twice the size of a wolf with large barrel chests. Their heads were somewhat flat with a large snout full of sharp teeth. They had large rounded ears with exceptional hearing. They were tan, with short fur, like that on the back of a deer. There were two distinct sounds they made. The most common was like the howl of a wolf. However, when close to a kill, the howl would be punctuated with bloodcurdling shrieks, like wild cries, cries of the need to kill, demanding and desperate. Their teeth were yellowish and were about three inches in length.
Heart hounds were beasts that were created along with the Boundary. They were the boundary's watchdogs. They could go in and out without being claimed by it, but they were tied to the underworld and couldn't go far from the boundary. When the boundary was up, they only came out at night. As the boundary weakened, they were able to stray further and further from it and came out earlier and earlier in the day. When the boundary failed, they were set loose, crazed and without purpose. Although very fierce, heart hounds were afraid of water. Carrying a bone of an underworld beast would confuse a heart hound, making them think you were one of their own.
The name "heart hound" came from two things. The first was because it was rumored they could find a man by the sound of the beating of his heart. The other was because they came at a victim's chest, his heart, when they killed (as opposed to the throat, like most beasts did). The heart was the first thing they would eat. If there was more than one hound, they would fight over the heart.