Siegfried

This is a version of Siegfried, a character of the nordic mythology. He slained the dragon Fafnir, and bathed in its blood, what granted him invencibility. But a little part of his body (in his back I think) wasn't covered by the dragon blood, and in that point was vulnerable, what eventually caused his death. The legend is similar to the greek legend of Achiles.

Hela

This is a version of Hela, the nordic godess of the dead. She has her own realm, the Helheim, where the dead habit. The Helheim is a region of the Niffelheim, the realm of eternal cold and darkness.