I've beaten the main story and I've since been working to fill out my medallium and complete quests, so I honestly forgot about the Infinite Inferno mentioned by Whisper at the start of postgame. I have no idea where it is! Does anyone else know?
It is in an old shack located in blossom heights, west of shoten temple, in the prayer's peak tunnel You need to go to Mr. Stone's house in Breezy Hills first to get the shack key.
I'm so close to being done with the main story. I would love to be able to finish that by this weekend, so I am hoping that I won't be super busy and not have that happen.