Complete the Vertigo Bestiary. Find the Depths Key. Both of these conditions can be fulfilled at the same time since the Depths Key only spawns in Vertigo waters. This includes strange whirlpools, in which you can catch Isonade. Once you complete this quest, all you have to do is talk...
Vertigo also has a unique variety of fish, and you can jump down from the shop into the cave to find a small place to fish and gather the unique fish to fill up your Bestiary. However, keep in mind that you cannot fill up your inventory because you won’t be able to purchase the ...