When you complete all quests from the first area (Stonewood), right after you unlock Plankerton, you will get a side quest called "Epic Troll Stash". To unlock this achievement, you have to complete that quest. To complete the quest itself, you need to play in 15+ areas.
The ultimate goal of the quest is finding the troll key to open a phantom chest. The phantom chest is a pink chest that looks like a chest-shaped ghost and is hidden somewhere on the map. You might find that chest at any time, which probably means that someone in the match has the side quest active, but only those with the active side quest and the key will be able to open it.
You get the troll key by whacking the troll that sometimes takes your BluGlo when you are about to grab it. You may have to try this a few times before you get the key. When you get it, you will see a message at the end of the match saying that you now have the troll key. After that, you just have to find the phantom chest and open it for its loot. The achievement pops at the end of the match, when you get your chest and quest rewards.
The phantom chest can spawn anywhere in any map (thanks Ztr7 for the supplementary info), and there are no marks on the mini-map or the map to guide you as to where it might be, so you'll just have to explore.
So, in a nutshell:
1) Complete Stonewood, get side quest "Epic Troll Stash"
2) Play in 15+ areas
3) Whack the troll until you get the troll key
4) Look for the phantom chest and open it
Update (Oct 31, 2017): Some people are having issues with this quest registering as complete and therefore can't unlock the achievement. Check the comments for a little more info (I don't know how to solve it though, but I'll keep my eyes open and update this as soon as I have new info)
Update (Nov 11, 2017): This is a known-issue, as acknowledged by Epic Games here: https://www.epicgames.com/fortnite/forums/bugs-issues/bug-re...
So hopefully it will be fixed soon!
Update (Dec 7, 2017): This has been fixed with today's update. Thanks Master Chiefing and Lythiris for the info. My guess is that it won't unlock retroactively though, you'll probably have to find the chest and open it one last time for the quest to register as completed.