After completing the first class quest on your world a new NPC called Garrick will appear in the tavern in Ravencrest.
Talking to Garrick he will provide you a map to the King Slime domain.
You can use this map with the Map Device to open a portal to the King's Domain.
Garrick's page will explain the quest, Kill the King.
Once you defeat the King Slime you can now receive the King Slime Domain Map as a global drop.
This will allow you to fight through the King Slime Domain once again.