As everyone else has pointed out, its something to do with his fire.
Also his SM Gimp attire could be a dead giveaway. Queen noted he is a 'torture-loving freak'. Perhaps King goes both ways. He's into torturing others and also enjoys self-torture. Specifically his power is a form of 'Pain Enhancement'. The more pain he receives the stronger he gets. Or it fuels his healing ability to the point of being near indistinguishable from regeneration. Or it enhances his durability.
Perhaps the flames are literally 'flames of pain', or that he lit the back of his neck on fire so he'd always feel pain and thereby be able to regenerate from damage and get stronger.
Maybe.
Also his SM Gimp attire could be a dead giveaway. Queen noted he is a 'torture-loving freak'. Perhaps King goes both ways. He's into torturing others and also enjoys self-torture. Specifically his power is a form of 'Pain Enhancement'. The more pain he receives the stronger he gets. Or it fuels his healing ability to the point of being near indistinguishable from regeneration. Or it enhances his durability.
Perhaps the flames are literally 'flames of pain', or that he lit the back of his neck on fire so he'd always feel pain and thereby be able to regenerate from damage and get stronger.
Maybe.