The Nine Worlds

What are the Nine Worlds in the Tree? See here. Apparently, no list survives (see here) so it has to be reconstructed, e.g., see here.

How Do We Get Nine?
2 primordial opposites, cold and heat, Niflheim and Muspelheim;
2 kinds of gods, Aesir in Asgard, Vanir in Vanaheim;
2 kinds of elves, light and dark;
2 other kinds of beings, men in Midgard, giants in Utgard/Jotunheim;
Hel.

But Hel might be part of Niflheim. Dark elves and dwarfs may or may not be identical. One name for the world of the dark elves is Myrkheim (see here; see also Mirkheim.)

At the Ragnarok:

the giants, the dead from Hel (led by Loki), Fenris Wolf, the World Serpent and Muspel will attack Asgard;

former inhabitants of Midgard will fight on both sides (we do, don't we?);

elves will remain neutral (?);

Vanaheim will survive and some of the gods will return from it later.