Helheim is built for Extermination only, and is not compatible with other gametypes. You can find ATN's Extermination game mode here.
Helheim is designed around the concept of 'reverse Valhalla'. If Valhalla was a Forerunner interpretation of heaven, Helheim is is that concept but applied to hell. Although in norse mythology Helheim is a frozen wasteland, I took the the theme in a direction more in line with Christian hell and built the map around a lake of lava. The structure of the map itself is designed to mimic a literal forge, with a giant anvil like platform in the middle and a pool of molten lava.
Helheim features two main areas - the central pit with the lava flow, and a U-turn platform that extends off over the cliff ledge. In the current build, a Dying Star spawns in the middle of the pit, and a damage boost spawns at the tip of U-turn.
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Special thanks to Ryouji Gunblade for making the death star energy portal above the map!