Corruption is a catch all term for what interacting with unnatural forces does to the mind, body, and soul of an individual. Some get weaker, some get gaunt, some appear to age, some lose their ability to feel compassion or any emotion for that matter. It really depends on the individual and can manifest in many different ways.
There is no explanation for corruption in the stories by REH, not in a specific sense. But every sorcerer, witch, wizard, and other user of dark arts has something wrong with them. This is represented in game by the corruption stat.
So in a roleplaying sense, you’ll need to come up with something that affects your character. It will be something noticeable by others, more and more so as the character delves too deeply. At least unless they take measures to hide it or distract from it.
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