The Ruins of Despair is unlocked after clearing Tower of Despair. It is in both Miitopia 2: Return of the Dark Curse and Miitopia: The Rise of Eterna. I haven't thought about ideas for this one as much yet as the Whispering Woods, however. The subarea consists of crumbled structures, mysterious runes, and tough ancient enemies! It is recommended that any team you send there is at least level 40.
Elite Goblin (non-boss variant)
Juvenile Snurp (non-boss variant)
Cursed Medallion (boss, based on Darker Lord's Left Hand)
The Ruins of Despair, along with the Tower of Despair, is all that remains of the time before Miitopia and are both considered by the game to be outside of Miitopia.