Oden 'hidden' island?

[Byte]

There's an island in oden's growth map which has hidden requirements for me. It's the one directly to the left of his 'Oden Two Sword Style Gun Modoki' special attack.

I'd appreciate it if someone told me what the requirements are, I know some chars like luffy's form changes had islands like that because of story progression requirements for the attack or form change but idk what I should be doing for this particular island.

[Byte]

Aight the requirement was the Kozoki Clan Emblem from the treasure log 'Death of a magician' or something

[Sniper King]

Right on, so the hidden islands are basically just coin requirements that it won't show you until you have the coin? I was wondering about those myself, good to know. Thanks OP.