The Axe of Doom was the primary weapon of Norbert the Nutjob along with Norbert's gigantic sword. The two blades are made of two completely different metals; a blackened evil looking blade and a gold blade. The Axe was lost during How to Ride a Dragon's Storm along with Norbert.
The Axe of Doom first appears in How to Cheat a Dragon's Curse, where it was used by Norbert the Nutjob in an attempt to decide the fate of Hiccup Horrendous Haddock III. Hiccup managed to catch the axe midair, a feat deemed impossible.
Norbert also fought Hiccup with the Axe of Doom during their confrontation on the mast of the The American Dream 2 during the sinking of said ship where it was being attacked by a Leviathorgan and was going through a thunderstorm, where the axe was struck by lighting. This resulted in the axe being lost, when it fell into the ocean.
Norbert used the Axe of Doom to fight, as well as decide the fate of his enemies. The axe had one half of the axehead made out of gold and one made out of rusty, black, metal with a skull on top. Norbert would spin the axe high into the air, and if it landed on the gold part, the victim would live. And if it landed on the black part, the victim would die.