That's weird. After I investigated the Dark Swamp it automatically triggered a fight with a purple swamp creature. Then the game said I had to visit Misty Abyss and you'll land automatically on a "!" ...
You sure you visited the Dark Swamp and not just explored Swamp..?
Did you try to enter the Misty Abyss in differents times cause it happen the same to me and i went in the night and when i came back at morning the "!" appear
