First off you will need to locate the item a rusted lockbox, this can be found randomly on the floor or a mob might have picked it up.
Item 'a rusted lockbox' is type container, with keywords 'small riveted steel lockbox rusted'. A rusted lockbox is in excellent condition. Capacity: 15 items. Maximum weight: 3 st, 50 pb. Flags: closeable pickproof closed locked It is closed. This lockbox looks like it has been stored somewhere damp for decades, and the rust covering the exterior does nothing to dispel this. There is a keyhole in the top of the lockbox, evincing a very stable and complex interior mechanism that prevents entry to the box' interior. This item is beyond your power to identify further.
Once located give the lockbox to a tired trapper and he will tell you the current weakness of a pernicious hodag. Find a weapon of that damage type and use it to kill a pernicious hodag, you will loot a 12-karat key from it. You will then need to locate a seaweed-covered chest which can be randomly found around Alyria, unlock it to obtain another key. The chest does not always have a key, so don't be surprised if it takes multiple tries. Take this key to Pirate's Cove onto the pirate ship and locate a locked north, unlock it go north and pick up the gold on floor.
Completing this will award the Mark of Plundering.
The booty can repop before the door locks again, allowing others to get the mark. However the room is solitary and will kick you out on repop of the booty (separate from the door locking). Therefore you will have to run back in to loot.