Svartalfheim (pronounced "SVART-alf-hame") is not one of the Nine Worlds, but it is mentioned by the dwarf Blitzen. It is the home of the svartalves, the Dark elves, a subset of dwarves.

Magnus Chase and the Gods of Asgard

The Sword of Summer

Blitzen mentions Svartalfheim when telling Magnus about svartalves.

Trivia

Locations (MC)
Nine Worlds: Alfheim | Asgard | Helheim | Jotunheim | Midgard | Muspellheim | Nidavellir | Niflheim | Vanaheim
Cities and Towns: Boston | Flåm | New London | Provincetown | York
Administrative Divisions: Connecticut | Massachusetts | New Hamshire | North Yorkshire | Sogn og Fjordane | South Carolina
Countries: Confederate States of America | Norway | United Kingdom (England) | United States of America
Continents: Europe | North America
Other Locations: Alderman Estate | Bifrost | Blitzen’s Best | Boston Public Library | Bridal Veil Falls | Cape Cod | Charles River | Charleston Harbor | Chase Mansion | Dorchester | Fadlan’s Falafel | Folkvanger | Fort Wagner | Ginnungagap | Hotel Valhalla | Loki’s Cavern | Longfellow Bridge | Lyngvi | Morris Island | Nabbi's Tavern | Provincetown Airport | River Oure | Sessrumnir | Svartalfheim | The Thinking Cup | Yggdrasil
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