He could barely eat because of his disfigured jaw. He suffered from rickets, hallucinations, and an oversized head. He was impotent and infertile. Charles II of Spain, king of one of the largest empires in the world, was barely able to talk or walk - all because his dynasty was so inbred.
Royal inbreeding caused mutations and birth defects that could be even worse than the already estimable genetic mutations from incest. In fact, Charles II of Spain's inbred birth made him more inbred than the children born of a union between brother and sister. And that's what ultimately explains Charles II of Spain's cause of death: by 35 all his hair fell out and he could barely walk, and before he turned 39 he died without an heir, plunging Europe into a bloody war as various nations vied for his crown.
Charles II of Spain's family tree was intentionally inbred. His father married his own niece, meaning that Charles's mother was also his cousin, and his dad was also his uncle. And that was just the beginning: the Spanish Habsburgs intermarried on purpose for over 200 years, making them among the weirdest royals in history.
With all these genetic issues, it's difficult not to wonder: what did Charles II of Spain look like? His prominent Habsburg jaw made it nearly impossible for the king to eat, and he constantly drooled. He was short, thin, and weak. But the family hired artists to make him look strong and healthy. Still, there was no hiding the outcome of centuries of inbreeding. Charles was the last Spanish Habsburg, a powerful dynasty that killed itself through inbreeding.