To begin the Goblin Diplomacy quest in RuneScape: Dragonwilds, you need to pick up the Bandosian Amulet from a goblin camp in the middle of a lake southeast from Bramblemead Valley. Defeat the goblins in that camp and head down the stairs. Your path will be blocked by thorny vines on the right, so you'll need to [[link]] chop them down to get ahold of the Reliquary.
Interact with this with E to receive the Bandosian Amulet. Similarly to the Restless Ghosts quest, this will automatically begin the quest but not give you many clues on what to do from here. You're challenged to find someone who knows something about the amulet, but that's it. Here's what you need to do.
How to complete the Goblin Diplomacy quest in RuneScape: Dragonwilds
To continue the Goblin Diplomacy quest, find Zanik at the edge of a cliff by a tent in northeast Bramblemead Valley. Zanik will explain that she recognises the amulet but you've only got one half and you'll need to find the second before she can be of any help. You'll also get a hint about the whereabouts, but it's not a detailed description since you're just told to visit a cave in the northeast.
After getting this clue, go to the Whispering Swamp and find the cave in the northeast with a clay deposit outside. You need to go inside, where you'll have to fight a level 3 Garou Druid. Keep your distance during this fight since this mob deals poison damage, and it's a pretty strong enemy if you've only just started the game. You can lure it out of the cave by attacking with a bow and then running, which [[link]] gives you more space to work with.
When you've [[link]] defeated the Druid, you'll be rewarded with the other half of the Bandosian Amulet. There is also a chest in this cave which contains some materials like Swamp Tar and Rune Essence which are worth grabbing while you're here. Go back to Zanik with both halves of the amulet to complete the quest. As a reward you'll get a Goblin War Banner, which you can equip as a cape to gain +2 Melee Defence, +2 Magic Defence, and +2 Ranged Defence.