Dragon Quest 5 Hidden dungeon
Successfully complete the game and make the end game save on your last saved file. Load it and use the “Ruura” spell to head to Evil Mountain. Instead of entering Evil Mountain, head directly south to what seems to be a poisonus swamp. You will enter a secret dungeon.
The image featured at the top of this post is ©The Legend of Heroes: Trails of Cold Steel III key art / Original