Important Detail - This may become unavailable at some point in the story (although this IS the last chapter of the game), and recommend you do this when you meet Arlecchino at the last Trinity door. Also, make sure to talk to Venigni when you see his icon next to Hotel Krat on the teleportation screen and exhaust all options. (Do that for every NCP) Firstly, to get this achievement you must answer all riddles correctly (the first one you can get wrong) to get the keys to open all Trinity doors. Luckily there is already a guide for that on here. In the last Trinity door, you will find Arlecchino, the riddle king. (Exhaust all dialogue options)
Secondly, you have to read the Collectible, “Scandal! V, the Tragedy Behind the Flamboyance”. This Book can be found in Lorenzini Arcade. In the Arcade, you will open up multiple shortcuts, and one of them will have a water fountain with a vendor next to it, in the same area you can find a bathroom with the book inside the last stall.
After reading the collectible, return to Arlecchino, and you will find a dialogue option "The truth behind the Venigni incident". After the reveal, he will give you the "Moon World Warrior Toy", return to Hotel Krat and talk to Venigni, and you will get the achievement!