Overview
POSSIBLE SPOILERS AHEAD
Obsidian Leaves, the Water Seal, and the Hidden Sea
Once you unlock Obsidian Leaves, a new area available to visit. Unlocking the new area requires an artifact that spawns in The Abyss, called The Water Seal. Once the artifact is used, you can access The Hidden Sea and start collecting Obsidian Leaves.
I haven’t found anything else online about this, so please comment any other info you have. 🙂
Here’s a screenshot of the artifact:
