What is the rarest skull in Sea of Thieves?

Could you please elaborate on the rarest skull found in the game Sea of Thieves? I'm curious to know which skull is considered the most elusive and valuable among players. Is it a specific type of skull that's harder to come by than others, or is it a unique variant of a common skull? Additionally, what makes this skull so rare, and what rewards or bonuses does it offer to those who manage to acquire it?