|— "Darkest Night"|
The Mace-Axe-Sword is first seen in the episode "Viking for Hire", bundled together with many other weapons Gobber forged. Their primary use was dragon-slaying, which no longer took place. When Hiccup asked Gobber to teach the Dragon Riders, he carted the weapons into the Berk Dragon Training Academy and brandished the Mace-Axe-Sword. All the dragons flew off, scared of violence.
A Mace-Axe-Sword is displayed on the wall of the wrecked Dragon Hunter ship, The Reaper. This suggests the Dragon Hunters may have made use of this weapon for dragon hunting, or it is possible it was a trophy from a fight with an opposing tribe. It is likely that the Mace-Axe-Sword isn't used for fighting but more for display because the weapon would be ridiculously large and heavy to wield in battle.
While Hiccup is dealing with the injury of his father in "Darkest Night", the other Riders tell stories about what Berk would be like if they had never left to go to Dragon's Edge. In Snotlout's idealistic fantasy, he started off as a "Weapons Tester" and would have been promoted to "Weapons Inventor". He would create the greatest weapon ever - the Mace-Axe-Sword. In his tale, he was also the awe of the village, with Gobber revering him and his weapon, proclaiming it to be one of the most beautiful things he had ever seen.
The Mace-Axe-Sword is a large and somewhat unrealistic looking weapon, as tall as or taller than Snotlout himself. It starts with a spear-tipped hilt, then becomes a jagged-edge sword. Midway along the blade is a double-headed axe. It terminates in another asymmetrical double-headed axe with a spiky mace cutting through the center.