Rift Achievement:A Barrel of Laughs
This puzzle is located at (3631, 2758) in the far north of Scarlet Gorge. To get here, cross the bridge high above Riverfell (where Scotty is) and turn right on the other side. Go until it dead-ends, then step off the cliff to the small ledge you see. Tucked under the planking is the puzzle.
This is a simple 3x3 slide puzzle like you might have done when you were little. It works on a grid where there is always one empty spot for an adjacent piece to move into. Shift the pieces around by clicking on them to move them into empty spaces until the left side matches the right side. Be sure to shift your viewpoint out behind and above you so you can see the pattern. Standing in the middle between the two grids you can see and manipulate the entire puzzle.
When you get it right you receive A Copper Key and a chest appears. Click on the chest to use the key, then click the chest again to loot it and get A Dusty Sack containing a blue item.
The puzzle can be completed by more than one person at a time, however, only the character who moves the last piece receives the rewards. The puzzle is repeatable but the reward after the first completion is only A Nearly Empty Bag.