This is acquired post game, after beating the Dragon.
Head back to Gran Sorean and you'll gain access to The Everfall, an endless abyss.
Within The Everfall there are many rooms, each containing powerful enemies and valuable loot. You'll meet Quince, a Pawn who'll give you the quest 'Fathom Deep', you will be required to gather 20 Wakestones, which contrary to most of the game, is quite easy here. Many floors have mini-bosses such as a Hydra, Dark Chimera, Evil Eyes and more which drop Wakestones.
Go from floor to floor, beating the mini-bosses, you'll usually earn at least 2 Wakestones (or the equivalent in shards). Bosses respawn every 3 days, but you shouldn't need to wait in order to get the number needed, it's quite easy.
Once you've obtained the 20 Stones needed, go back to Quince and you'll complete the quest. A portal will open and this will allow you to escape The Everfall. Jumping into the portal will make you unable to do anything else for the remainder of the game, this is the point of no return.
So make sure you've done whatever else you want to do.
You can view floor plans and various info about The Everfall here