Below is a walkthrough on the prologue in Voice of Cards: The Isle Dragon Roars.
Dank Cave
You’ll begin Voice of Cards: The Isle Dragon Roars in the Dank Cave. When you’re able, move forward towards the northern door. Select to either push or pull on the door’s handle. It does not matter. Continue north until you flip over a card bearing a chest on it. Please note that there is nothing else of interest in this cave besides the chest. You might have an enemy encounter or two but it’s not worth trying to level these characters up! (You’ll see why after you begin the hunt for the dragon!) Finding this chest will trigger a fight. Defeat the enemies then select the Simple Key to open the chest and receive Medicine (?) aka Royal Treasure. After the dialog, you’ll be teleported out of the cave to the castle!
Click here for the next guide on Chapter 1!