I don't know if this should be here it's not really a tale but I don't know where to put it,
also I didn't find anything about this when a search in the forums so I don't know if anyone talked about this before. Okay I'm really tired and stupid I guess ^^, I just searched on google and found people talking about it, it was for a gamescom event apparently and they just left the skulls afterward... (Old post that talk about it) Damn I thought I had found a easter egg or somehing. I'm kind of disapointed now...
I was doing an OoT's voyage when I found a skull painted blue and marked with the gold hoarder key inside a cave of Devil's ridge but I didn't think much of it at first, then on Crook's hollow I found a red one so I looked on the other large island in the ancient isle to see if there were more of them and sure enought I found a green one on Plunder valley and a yellow one on Discovery ridge.
The gold hoarder key of their forehead seems to point in four different directions I tried to follow one but didn't find anything.
I wonder if there are more skull or stuff marked with the order of souls or merchant alliance's emblem in the two other region, I'll try and search later.