Twilight of the Gods
The “nine worlds” are referenced by Snorri Sturluson, but they are never enumerated clearly. For Twilight of the Gods, the Nine Worlds are:
Asgard, the home of the Aesir, the new gods
Vanaheim, the home of the Vanir, the old gods and wise spirits of the world
Alfarheim, home of the Ljosalfar and their King and Queen Freyr and Freyja
Midgard, home of human beings, alfar, dvergar, huldufolk, trols and skraelings
Jotunheim, realm of the Giants who are like gods themselves
Niflheim, realm of the storm-giants and frost-giants, reached by one of the three roots of Yggdrasil. Near the entrance lies Mimir’s Well, where Odin sacrificed his eye for a drink of wisdom.
Nithavellir, the Hall and refuge of the dvergar Sindri
Svartalfarheim, the underground realm of the dark elves and dvergar
Hel, also under the earth and reached by one of the three roots of Yggdrasil, the realm of the dead ruled by a goddess of the same name
Muspelheim, the realm of fire, from which Surt and his legions of fire giants will march at the end of days. The gate to the realm of the fire giants opens only when Ragnarok begins
…nine with the addition of Nithavellir, which is like a world unto itself, but is also technically a single home.