Black Mask

The Black Mask is the one plot item in the game that is entirely possible to obtain before you have any idea what it's for. You need it to bypass the doppelganger on the first floor of the Mysidian Cave, but it doesn't hurt to get it early.
Compare Black Mask to other Key Items
Treasures: Containing Black Mask – 1 shown
The map where the treasure is found
Map
The specifics of where this treasure can be found
Notes
The monsters guarding this treasure, which will attack when the treasure is opened
Guarded By
Tropical Island B5 Open area central room -
White Mask Crystal Rod

Related

The Tropical Island (Optional)

The Dragoons of Deist
Page 2
The party may explore the Tropical Island where they find the mysterious Black Mask.

Mysidia

The Ultima Tome
Page 2
The magical town of Mysidia reveals the uses of the White and Black Masks.

The Mysidian Cave

The Ultima Tome
Page 3
With the Black Mask from the Tropical Island in hand, the party can delve into the Mysidian Cave to find the Crystal Rod.