You can access different realms via the Conjurer at the spawn.
Monster levels range from level 1 to level 25.
Guardstone Boss Map
Headless Landing Map
Monster levels range from level 25 to level 35.
The World Boss in Headless Landing is Bonemother.
Headless Boss Map
Monster level range from level 35 to level 45.
The World Boss in Faivel is Shatterspine.
Faivel Boss Map