There is a reason why there are only twelve Zodiacs. You see when the Gods grew tired of making sure the mortals didn't go extinct, they created glorified babysitters to take their place, and many rules to ensure the Zodiacs didn't attempt to overthrow them-needless to say, it blew up in their faces... literally. Ophiuchus was the reason why everything went to shit, he broke rule number one: don't get married to a mortal, but most of all, don't bang said mortal and reproduce. Guess what? He did just that, and the rules were very clear-if the Gods found out you broke that rule, they would slaughter your entire family... while you watched. That is exactly what they did to him, and he went mad with grief and anger and set out on a suicide mission to wipe the Gods out, and exact glorious revenge on those who wronged him.