See the Darkened Sea - TDS Collectibles Achievement
For this achievement you'll need to complete the collectible achievements for four different zones. Katta Castrum: Deluge (Secrets Beneath the Ocean), Tempest Temple (Riding the Storm Out), Brother Island (Island Fragments) and Cavern Collectibles (Caverns of Endless Song). There are 4 collection sets per zone which will make a total of 16 collection sets that you need to complete.
This is one of two achievements that is required for the Master Scavenger of The Darkened Sea achievement which rewards the player with Hoarder's Belt Pouch (38 slot 100% Weight Reduction Bag) and the Trophy Collector AA. The Trophy collector AA is a must have for those looking to min/max their characters.