The Broken Mirror Darkly - TBM Collectibles Achievement
For this achievement you'll need to complete the collectible achievements for two different zones, Relics of Sul (Crypt of Sul) and Relics of Decay (Crypt of Decay). There's four collectible sets for each of these zones so you'll need to complete 8 sets in total.
This is one of two achievements which is required for the Master Scavenger of The Broken Mirror achievement which rewards the player with a 38 slot bag and 1 extra Trophy Slot, a powerful upgrade for your character if you min/max.
Required Achievements:
Relics of Sul (Crypt of Sul)
Relics of Decay (Crypt of Decay)