This is definitely one of the more obscure quests in the game, I believe this is initially activated by losing a certain amount of fruit from falling into the thorns.
Once you see a mini cutscene where several shadowy figures break free, you can begin this quest.
You also need the crimson ballFollow this route from the post office.
To begin the quest speak to the crimson guard on the right, and then travel to the monkey statue.
Go down the slide next to the mushroom soil to enter the statue.
Simply complete the relatively simple pinball segment inside the monkey, then return back to the crimson guard, speak to him again.
Your reward is the boon of plenty and a shiny achievement.