Hello, sorry for the dumb question but I keep trying to find "Diabosite" that you need to build the prison but I legit can't find it anywhere :( could someone help?
If I remember correctly, you have to complete the "Rat Problem" and "Rats!" quests for Harold/the Bareshade tavernkeeper, which will unlock the tavern's basement for you.
After that, you have to keep exploring the area close to the entrance until you unlock the mine. It's a random event though, so you gotta keep trying
Hope this helps!