Young Souls

To obtain the Silver master key, you'll need to kill Balkin in the Full Moon zone in the Drowned Caverns. Once obtained you'll need to open all silver chests in all 3 main areas on the map.

Luckily these are denoted on the fast travel map with indicators in every quick travel zone. You should do this prior to the 4th zone which will have you finishing the story.

Feel free to comment or DM with any questions, comments, or concerns!

~ Slayer

You obtain the Silver Key in the Drowned Caverns (second main area). The Silver Key is behind the level 7 door Full Moon and is the reward for defeating the boss in that room.