In Clair Obscur: Expedition 33, the Old Key is one of the important items you can obtain early in the game, though you will only be able to use it much later in the story.

To get the Old Key, you need to reach the harbor during the festival celebration. There, you will interact with several expedition companions who will hand you festival tokens. You will have one token right away. With this token, you need to find a woman named Colette, who is dressed in blue and stands in front of one of the booths on the right side. Speak with her, and she will exchange your token for the Old Key. It's worth noting that Colette will not reveal where the key should be used, so its purpose will remain a mystery for some time.

You will be able to use the Old Key only after reaching an area called Old Lumière. This will happen approximately 10–15 hours into the main story. In this segment of the game, you will control Malle and a new character named Verso.

After arriving at Old Lumière, move through the streets until you find a Rest Point flag located on the left side. From the flag, move straight ahead, then turn right, passing by a giant sword stuck in the ground. Continue through a broken doorway and immediately turn right again. Here, you will see an old building you can enter.

On the upper level of this building, you will spot a glowing item — a Polished Color Catalyst. However, if you jump down into the building through an opening, you will encounter locked old doors. This is where you will need the Old Key to open them.

By unlocking the doors with the Old Key, you will find the Fracture Survivor’s Journal inside. This journal is important for making progress toward earning the trophy or achievement called "Follow the Trail" in Clair Obscur: Expedition 33.