Hel is Loki's daughter and sister to the wolf Fenrir and serpent Jormungandr and presides in the realm with the same name (the underworld). She was appointed by Odin to be the ruler of Niflheim. Warriors who fell in combat did not become her subjects but went instead to the hall called Valhalla to live with Odin. In early Norse mythology, Hel was also the name of the world of the dead.
It is said that once the gods found that these three children are being brought up in the land of Jötunheimr, and when the gods "traced prophecies that from these siblings great mischief and disaster would arise for them" then the gods expected a lot of trouble from the three children, partially due to the nature of the mother of the children, yet worse so due to the nature of their father.
After this discovery, Odin sent the gods to gather the children and bring them to him. Upon their arrival, Odin threw Jörmungandr into "that deep sea that lies round all lands," Odin threw Hel into Niflheim, and bestowed upon her authority over nine worlds, in that she must "administer board and lodging to those sent to her, and that is those who die of sickness or old age."
In this realm Hel has "great Mansions" with extremely high walls and immense gates, a hall called Éljúðnir, a dish called "Hunger," a knife called "Famine," the servant Ganglati (Old Norse "lazy walker"), the serving-maid Ganglöt (also "lazy walker"), the entrance threshold "Stumbling-block," the bed "Sick-bed," and the curtains "Gleaming-bale." High describes Hel as "half black and half flesh-colored," adding that this makes her easily recognizable, and furthermore that Hel is "rather downcast and fierce-looking." She is also said to be the only one capable of controlling the dragon Nidhoggr.
After the death of Baldr at the hands of Loki, the goddess Frigg asks who among the Aesir will earn "all her love and favor" by riding to Hel, the location, to try to find Baldr, and offer Hel herself a ransom. The god Hermóðr volunteers and sets off upon the eight-legged horse Sleipnir to Hel. Hermóðr arrives in Hel's hall, finds his brother Baldr there, and stays the night. The next morning, Hermóðr begs Hel to allow Baldr to ride home with him, and tells her about the great weeping the Aesir have done upon Baldr's death. Hel says the love people have for Baldr that Hermóðr has claimed must be tested. This test being that if all the things in this world, living or dead, mourn for Baldrr then he will return to the land of the living. If one refuses, then he will remain in her domain.