What is the rarest quest in Sea of Thieves?

I'm curious, could you elaborate on what is considered the rarest quest in Sea of Thieves? Is it a treasure hunt that leads to hidden riches, or perhaps a challenging voyage that tests the mettle of even the most seasoned pirates? Are there any specific conditions or prerequisites that need to be met in order to unlock this elusive quest? And most importantly, what kind of rewards does it offer to those brave enough to embark on this journey?