An infinite number of realms make up the Netherworld. Each realm is very different from the next because they represent the feelings people had when they died. Many Folks, which rerepresent the souls of the dead, inhabit each realm. The strongest of them in each realm is called a Folklore.

To visit a realm from the real world, a memento is required as an offering at the Henge in Doolin.
There are currently six playable realms in Folklore but there have been suggestions from the downloadable content that there are many more that Ellen and Keats have yet to visit. In Chapter 6 you have to fight a Folklore in the Corrupt Village, which could arguably be the seventh Realm.

List of Playable Realms

Navigate the Realms

Faery RealmWarcadiaUndersea CityEndless CorridorHellRealmCorrupt VillageNetherworld Core