Helheim is the final location in Tomb Raider: Underworld, and it is where the final events of the game unfold, as Lara and Natla co-operate to open the gates to the Midgard Serpent. It is part of the Arctic Sea levels.
After opening the underwater entrance to Helheim, Lara swims into the giant castle, and finds the gates. She also meets Natla, who performs the ritual to open the gates, and Lara crosses the pool of eitr in front of the gates to open the gates with Mjölnir. Once inside, Lara finds a figure staring at an eitr waterfall. Lara calls out to the figure, revealing her as Amelia Croft. However, Amelia had fallen into the eitr sometime before, and has become a thrall. After defeating the thrall, Natla gloats about the irony of the situation, and reveals to Lara that she had used Lara all along. After threatening Natla, Lara is attacked by the Doppelgänger, who prepares to kill her, but Amanda intervenes. While Amanda deals with the thralls, Lara chases Natla into a massive cavern with a gigantic mechanism in the center of it. Natla reveals that the Midgard Serpent is a metaphorical name for the system of tectonic ridges around the planet, and that Helheim stands on its weakest point. By activating the machine, Natla plans to exploit it to bring about the Seventh Age, but Lara manages to disable the machine. Natla attempts to hold it up, but Lara throws Mjölnir at Natla, throwing her straight into the eitr. The machine collapses, as Lara and Amanda contemplate their current situation. Lara gets an idea - she saw a dais as she was disabling the machine, and she and Amanda attempt to activate the machine. In one last show of trust, Amanda and Lara manage to use the portal, returning to Nepal.