[ See how many interesting facts about worlds you can live if you ask questions about other people, Senku!!

Especially since it's not like Inigo minds talking about this. ]

It's our bloodline. One of my ancestors from really long ago made a pact with the Divine Dragon of our world. Ever since then her blood has been running through our family's veins. [ He points to his eyes - or more specifically, at the one that doesn't look 100% normal, an odd marking in it rather than a regular pupil. ] It's why I have this eye.