The Mystical Key and Polar Opposites quests come immediately after each other in Crimson Desert. You will have just won an arm wrestling match in Hernand tavern and helped some random villagers clean their chimney. Now, though, you have to take a mysterious key and use it to unlock a room in Hernand Castle.
After that, you'll have to use the Axiom Force ability to move some dials and complete a circuit. All that said, here's how to finish both quests so you can keep progressing through Chapter 1.
