In God of War (2018), nine realms are linked by the World Tree.
The World Tree, or Yggdrasil, connects the major locations of the game's Norse mythological setting. Kratos and Atreus travel between realms using the Bifröst, a magical travel system accessed from Tyr's Temple in the Lake of Nine.
The nine realms include Midgard, Alfheim, Helheim, Muspelheim, Niflheim, Jötunheim, Asgard, Vanaheim, and Svartalfheim. Not all of them are fully accessible in the 2018 game; some are unavailable or only referenced, with additional realms becoming more important in God of War Ragnarök.
The number comes from the traditional Norse-mythological idea of nine worlds, although the game's presentation adapts that material for its own story and gameplay. The Lake of Nine is a location in Midgard, not a tenth realm.