some guy from reddit:
It's all head canon and overthinking, let's see until the scans and further spoilers but I think the shadow might be related to cursed pirate Davy Jones who Foxy and Robin mentioned in Long Ring Long Land who is a reference from irl myth/metaphor of
Davy's Jones Locker, the sailor's devil. This is would connect the Legend to Blackbeard and Xebec. Davy Back Fight originated in Hachinosu island where Xebec built his crew and Blackbeard is now.
Due to the pride and honor inherent to the game, pirates are expected to follow these conditions to the heart, and gladly defect from their original crews. To emphasize this, games are traditionally inaugurated by throwing three coins off the nearest coastline - a tribute that not only honors Davy Jones, but symbolically promises him the soul of any pirate who betrays the game's spirit.
Also, Robin:
"He is a cursed pirate from ancient times that lives under the sea. It's a legend, he also takes takes all the ships and treasure that sink to the sea floor.