Hermóðr

Hermodr (Old Norse: "Hermóðr" - “Spirit of War”) is the messenger of the gods in Norse mythology, and is the fastest among the Æsir. When Baldr died, Hermóðr volunteered to go to Niflheimr to ask Hel to release Odin's beloved son. Hermóðr was said to have galloped on the horse Sleipnir for nine days and nine nights.
Journey to Hel.
Upon reaching the dreaded world of ice and mist, Hermóðr encountered the female jötunn Móðguðr (Old Norse: “Furious Battler”), guardian of Gjallarbrú, the bridge which spans the river Gjöll (Old Norse: Gjǫll - “Noisy”), which separates the realm of the living and the dead. When Hermóðr stated his business as to why he wished to enter Hel, Móðguðr let him pass. He finally met with Hel, and begged her to release Baldr. Hel eventually agreed to his request, on the condition that all of creation weep for Baldr.