On Citadelle des Morts, you can trigger a special cutscene with the character Maya, where she seeks revenge for the death of her brother Nathan. To access this cutscene, you must be playing as Maya on Citadelle.

Head to the dining hall and approach the following door. Interact with the radio next to it to proceed.

Once you interact with the radio, a voiceline will play. After it’s finished, you can interact with the door, and you’ll be teleported to a special area. In this area, you’ll need to kill a large number of vermins and other enemies. Once you’ve completed this, you’ll receive a key to open the locked door. This will trigger the mentioned cutscene.