Bottumless dungeon.
Look for secret pathways.
Viewing post in (WIP) Hailey's Treasure Adventure(+18) comments
When you reach to the platform where the zombie is, there is a little legde on the right side where you normaly would go down, instead jump across and go prone, there is a hidden entrance that leads to a goblin hideout where the key is also located. Not sure if you need to find all the missing posters first, before you can get the key.