To get this achievement you must save every camp, save the farm, save the quarry. Once everything on your map is blue and not red, go to the final battle by pressing
when you have Darkhelm highlighted on the map.
The battle has several objectives. There are three Shamans and one group of archers that you have to eliminate. As soon as you enter the city, you'll see a Champion right in front of you. Kill him in whatever manner you choose. Doing so will grant you two dragon runes. With those runes, go to Eye of Asgard view by pressing
and use the dragon to take out one of the Shamans. Continue into the city to find another Champion and repeat the process to eliminate another Shaman. Move around to the back of the city and take out the archers. This leaves only one Shaman, so attack him by eliminating the glowing pillars that surround him. Once all five are destroyed, the Shaman will be vulnerable and you'll be able to take him out.
Once the last Shaman is dead, Hel's Champion Drakan will appear. If you have some axes, you can damage him from a distance before closing in and finishing him off. The battle is basically the same as a normal Champion battle. After this boss battle this achievement unlocks.