Corruption is an attribute, like Strength or Wisdom. It starts at 10. (Paladins have an inherent −2 modifier. Tieflings have an inherent +1 modifier.)

Corruption is not the same thing as an evil alignment. Rather, corruption represents otherworldly, supernatural evil—a shift in one’s mentality and spiritual tendencies, toward an inhuman outlook, of a sort that humanoids typically understand as being evil. (Note that deviations from the average in both directions represents distance from human mentality. This means that paladins are more different from normal humans, in this regard, than most tieflings are.)

Lower corruption scores represent paladins, saints, champions of light, or even celestial beings. Higher scores represent servants of evil.

Losing corruption is very difficult. Gaining it is easier.

You can gain corruption by being exposed to supernatural evil. Usually, only voluntarily choosing to make use of evil powers results in corruption. However, particularly strong corrupting influences may affect you involuntarily, especially as you spend more time in their presence.

Corruption makes you more susceptible to influence from (and, in the extreme, control by) fiends and other evil powers.

While it’s possible to ignore it, those who become aware of their corruption typically choose either to resist it or to embrace it. Both paths have their benefits and their dangers.

Embracing corruption means that one stands a greater chance of wielding or controlling evil entities or influences, rather than being controlled by them; and there is power—magical, spiritual, and even physical—to be drawn from corruption itself. But to embrace corruption is to become more susceptible to its advance; and there comes a point along that path when one is no longer a free-willed mortal, but an instrument of evil.

Resisting the effects of corruption is difficult, and it means giving up what is potentially a great deal of power. And the choice to resist is no guarantee that one will successfully avoid corruption’s influence. But it does improve one’s chances; and resisting the temptations of that power is the only option that allows one to retain the hope of cleansing one’s corruption entirely.