You do spawn with a fighting style, unless you mean learning it from a master.
Aurem reference I presume?
Better idea since Davy Jones likely won’t be added:
By hunting down and killing every shark in the game, you acquire this badge (this does not include sea monsters). The reward could be a shark tooth necklace or smthn.
Monster Hunter
Same as above except by killing every sea monster found in the war seas (or just the monsters found in a specific part of the war seas, like the bronze sea. Perhaps a different badge for each sea? Dark sea monsters not included). This could give you a special item made from the parts of all the monsters you killed, such as a lantern or a special ship part.
The Abyss Stares Back…
Acquired by killing every dark sea monster. Would likely give a title.
Good ideas but quite unnecessary, especially since all of these are easy to obtain.
It would be worthwhile to make a badge for something thats hard to obtain/do.
