With the Royal Edition of Final Fantasy 15, the ruined city of Insomnia has a very different look. If you explore the game from Chapter 14, you will notice that the path to the final boss has completely changed.
The ruined city of Insomnia is now much larger than before, and the city’s underground subways are home to the Glaives base. As you walk through this base, you will come across a man who is embarrassed in front of the elevator doors… He has lost the key ! If you are also looking for the key in the ruined city of Insomnia, you will find on this page the exact place where you can retrieve it. This key with a name, the Underpass Master Key, it opens many accesses in Insomnia.
Final Fantasy XV : Where is the key to the glaives headquarters in Insomnia ?
You will be able to get the Underpass Master Key of Insomnia’s underground tunnels by going to the surface of the ruined city. You must go precisely to the rest point  » LR Citadel Station  » (look at the screenshot below), you will find in the corridor of this resting place a bright red object on the ground, it is the key of Insomnia / the Underpass Master Key. In this same corridor, you will find the elevator that allows you to get to Insomnia’s glaives headquarters faster.
