Hel's terrible realm of Helheim was located in the depths of Niflheim, beyond the impassable river, Gjoll. Any who enter Helheim, including the gods, can never leave again. The entrance is guarded by the monstrous hound, Garm, and the undead giantess, Modgud, who demands a toll in blood before passing. Hrimgrimir ("frost shrouded") and many other giants also guard Hel's gates.

“Trembles Yggdrasil’s 
ash yet standing;
groans that aged tree,
and the jötun is loosed.
Loud bays Garm
before the Gnupa-cave,
his bonds he rends asunder;
and the wolf runs.”

-- The Poetic Eddas, The Vala´s Prophecy