Hel (or Helheim) is the realm of the dead in Norse Mythology and the name of it's ruler, Hel, daughter of Loki and Angrboda. In the show and promotional material, half her face is that of a skull and she is wearing a black cloak with a hood. The name shares the origin of the English word for "Hell", and means "hidden", "concealed".
Hel Lokisdottir is the goddess of death and graves. She cares for the ones that have died of age or sickness and sends "wolfish murderers," "men who seduce another's faithful wife," and oath-breakers to Nástrand where the lindworm Nidhögg dwells. Half her body is that of a pale woman, the other is "corpse-blue," practically rotten. When the Swedish king Dyggvi died, Hel made love to his corpse and took him as her concubine in Helheim.
The Seer tells Lagertha that the shadows that haunt her dreams comes from "Hell's hall". Hel also appears in a single frame during the opening credits, when the thunder rolls. King Horik compares King Ecbert's two-facedness to Hel. In Full Moon, Rafarta accuses Floki of having taken the expedition to the land of Loki and that Hel lives in the hot springs of Flokalundur.