The 40k corruption point effects would be good inspiration imo. Not necessarily the way you get them (ranging from 0-100 corruption pts) but the effects are nice and dark.
For example, every time you receive your Con in taint points you gain a taint trait - sudden outbursts of aggression, sensitivity to light, craving raw meat etc. Once you've received X number of traits you've become a ghoul (X could also be your Con value). As your Con goes up the amount you need to get a trait also goes up. Resistance tests to avoid getting traits could also be used, so that you could end up with a high number of taint pts without actually accruing any traits and thus not becoming a ghoul (as in the above it's the number of traits that makes you a ghoul, not the taint pts).
Grey Wardens either get a bonus to the resistance test, or never suffer taint traits. So you can still keep score of how f'ed up you are.
As for when you get taint pts, that could be GM based, and/or you could have special 'infect' stunts that blight tainted creatures/darkspawn can perform that has a chance of forcibly giving you taint pts.
Hellebore





