The Lost Tribe

Walkthrough
Talk to all of the people in Lumbridge, including the duke, the Cook, Hans, the Priest, and Bob, as well as anyone else you find. One of them will tell you that they saw a goblin with huge eyes in the castle basement. Report this to Sigmund. He'll tell you to investigate this.

Go down the ladder in the kitchen and look for a blocked tunnel. Use your pickaxe on it. Light your light source, then squeeze through, grab the brooch, and turn back. Otherwise you'll fall into a pit. Show the brooch to the duke and Sigmund. They will tell you to go to the library in Varrock.

Show it to Redlo, the librarian. He'll tell you to search the bookcases for a book on goblin history. Read it to see that the symbol on the brooch translates to "Dorgeshuun, goblins of the strong spears". Keep the book for the remainder of the quest, since it's very important.

Talk to Redlo again, then talk to General Bentnoze and General Wartface in Goblin Village. They'll argue about minor things just like they did in Goblin Diplomacy, but eventually teach you the goblin victory dance and ceremonial bow emotes.

Go back to Lumbridge and talk to the duke. He will tell you to investigate the caves further. Go back into the tunnel. First, go south and use your pickaxe on the tunnel to unblock another tunnel leading to the main cave (this helps a ton during Tears of Guthix). As you go through, read the symbols on the rocks. Then, translate them with the goblin history book and follow the instructions. For example, if it translates to "Goblins of the East", go east. If you go the wrong way, one of two things will happen:

1. A rock will fall in front of you or on top of you (if you're lucky).

2. You'll fall down a trapdoor and end up in the main part of the Lumbridge swamp caves. If you're using a candle, you're pretty much doomed, since you need a light source to stay alive, and the candle will constantly cause gas explosions.

When you reach the end, you'll find the Dorgeshuun tribe. Do your goblin bow in front of Mistag. He will tell you that the Dorgeshuun are actually peaceful and the Lumbridge kitchen thing was an accident. You'll also learn about the goblins' backstory. Mistag will offer to take you back to Lumbridge. Agree.

When you tell the duke about it, he won't believe you because some silverware was recently stolen from the castle basement. Pickpocket Sigmund and unlock his chest to find some HAM robes (keep them, as you'll need these for Death to the Dorgeshuun, the sequel quest). Tell the the duke that Sigmund is part of Humans Against Monsters. Then go to the HAM lair (the trapdoor is directly west of the general store, pick the lock). Search the crates to find the stolen silverware. Give it to the duke. He will fire Sigmund.

The duke will give you a peace treaty. He will sign it and tell you to get the leader of the Dorgeshuun to sign it as well. Go into the tunnel. This time, you won't have to navigate through yourself because you'll see a new goblin there that will take you to the mines. Talk to Mistag. He'll get the leader to sign it in the Lumbridge kitchen, with you and Mistag as witnesses.

Reward

 * 1 quest point
 * 3000 mining experience
 * The ability to use the goblin bow and dance emotes
 * Access to the Dorgeshuun mines
 * A ring of life