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I want to have a hero with horns AND a halo. Is that possible? If so, how do I go about raising my purity and evil?

I don't think you can have horns and a halo, but you can be PURE EVIL! (Cue the evil laughter!) You can increase your purity by buying and renting out those houses with low rent (a negative number in the "Adjust Rent" option), or by buying business and lowering the prices (a negative number in the "Adjust Prices" option), as well as by eating fuits and vegetables, rather than meat or meat products. You can increase your evil by slaughtering innocent villagers, stealing, and by sacrificing people to the Temple of Shadows. To do this, you must gain entry to the Temple by speaking with the monk in front, who will give you 'crunchy chicks.' Eat them all, and you'll be admitted. You'll also earn extra evil if you sacrifice your spouse. Bear in mind, however, that if you have a child with this spouse, the child will be taken away after you sacrifice your spouse. Please note that you can't kill children, but you can extort and terrify them.

Actually they depend more on the purity/corruption rather than morality. You need corruption for Horns and you need purity for Halo. Aswell as a good/evil alignment.

I think halos are reserved for pure and good. Maybe just good. Anyway, you can be pure and evil. You'll have evil, gray skin, but you'll still look good. As long as I'm going to be evil, though, I'll be corrupt and have the whole demon thing going on for me.