Ras Al Ghul
Ra's Al Ghul is a fictional character, whose name when translated means 'The Demon's Head.' He was created by artist Neal Adams and writer Denny O'Neil in Batman #232's 'Daughter of the Demon.'Ra's Al Ghul was several centuries old before he ever encountered the Batman. Replenishing his aged, injured, or dead body with a bath in a 'Lazarus Pit,' Ra's has survived through the ages by accumulating wealth, knowledge, and power. He rivals the Dark Knight in hand-to-hand combat and sword fighting. He has a devout following that blindly does their master's bidding, and employs the 'League of Assassins' who consist of some of the most dangerous men and women on Earth. However, once a being uses a Lazarus Pit, they emerge insane albeit with the temporary advanrage of superhuman strength.
Al Ghul's goal is to cleanse the world by paring down the human race to a more manageable size. His goal, and the fact that he has the means to achieve it, bring him in conflict with Gotham's Caped Crusader. He usually tries to assault the world's human populace with a biological menace, like a plague.
Al Ghul is Batman's most formidable foe. Not only can he physically and mentally challenge 'The Detective,' as Ra's always refers to him, Ra's Al Ghul has deduced that Bruce Wayne is Batman. Ra's first met Batman by surprising him in the Batcave, seemingly to enlist his aid in rescuing both Al Ghul's daughter, Talia, and Dick Grayson, the first Robin. However the whole affair was a charade orchestrated by Ra's to test Batman, whom Ra's views as a worthy successor to the Demon himself.
Talia, the daughter of the Demon, is also one of Bruce Wayne/Batman's love interests, perpetually addressing him as 'Beloved.' Talia has recently had a falling out with the Demon and left to run LexCorp for former US President Lex Luthor, before selling the company to Bruce Wayne to aid Batman & Superman's victory over Luthor.
Ra's blames his relationship with Talia on the Darknight Detective, and had Ubu, (the title for Al Ghul's primary servant,) stage an assassination attempt on Bruce Wayne's longtime ward Dick Grayson, shortly before Wayne officially adopted the now fully grown Grayson.