"I'm the dog, the big bad dog. I'm the dog, the bounty hunter."
- – Ozzy Osbourne, making a surprisingly accurate description of Karanak
Karanak is a Flesh Hound in the service of Khorne. Unlike normal Flesh Hounds, he has three heads which apparently allow him to track his prey's trails in space and time, as well as their thoughts. Basically, if for some reason this super-Cerberus decides to come after you, you're pretty boned (Unless you’re Shalaxi Helbane). Khorne cares not from whence the blood flows, only that it flows, but he is still a god of honor. Offer him a skull from an unworthy kill in tribute, or just piss him off, and he will send his favorite dog after you.
On the Tabletop
Karanak acts as Khorne's other named Herald. Karanak gets to reroll all to-hit rolls and to-wound rolls against a character selected at the start of the game, grants his entire unit rage (hilarious on Bloodcrushers if they can make it in) and causes any psykers within 12" of him to peril whenever they roll doubles. He has Scout, like every Flesh Hound.
Bizarrely, he isn't part of the Khorne Daemonkin codex, despite his buddy Skulltaker making it in. It's anyone's guess as to why.