In the latest zombie map DLC for Black Ops 2 called ‘Mob of The Dead’, there is a secret weapon and Easter egg called Hell’s Retriever. Hell’s Retriever is basically a thrown tomahawk weapon that deals pretty decent damage to zombies in a line. At the lower levels it can one-shot an entire horde as it damages while flying at the zombie horde, and also while it bounces back to your hand.
Obtaining the weapon is a bit tricky, as there is a special procedure to complete involving three wall marking and zombie sacrifices. Obtaining the weapon also awards you the Feed the Beast achievement.
To start off you need to find one of the three dog head wall markings that are hidden in Mob of the Dead. The first marking is found on the second floor of Broadway, you will come to a place that has some safety railings. Check the walls around here.
The second dogs head is found when you enter the docks from the prison underground and turn right at the door. The head is on the wall.
The last mark is left of the roof access through the Infirmary, and near the part item in the locked case.
Once you find a mark, stand by it and wait for a zombie to appear. When one finally does, kill it near the mark. The mark comes to life and a demonic dogs head replaces it. The head scoops up the corpse and eats it. You need to feed the dog a total of six zombies before it goes away for good. Do this for all three of the dog heads.
When you’re finished head over through the showers and open the doors to the citadel tunnels. There will now be a missing wall here with three dog heads over the gap. Enter through the breach and you will find the tomahawk Hells Retriever suspended in a beam of red light here.