This dimension was directly named after the Underworld in Norse mythology.
Helheim could refer to: The forest that is the medium for the "Trial of Evolution" The planet that was given the codename Helheim by Yggdrasill Corporation where which the Helheim Forest was discovered. The sentience of the Helheim Forest who took on the name Helheim as a representative description of his being. The newly-born planet where Kouta Kazuraba sent the Helheim Forest and the Inves after acquiring the powers of the Fruit of Knowledge.
Related Post : Source : wikipedia / metalheroes.wikia