Instructions
Play through the game until you reach the level called "The Crucible." Check your logbook from the "Start" menu and you'll see a "purity/corruption" level indicator. A marker will be balanced between the two extremes, depending on your behavior up until this point. To increase your corruption level you can raise the rent on your properties to 100 percent and sleep for a couple of weeks to get your tenants angry with you. You can also choose the more corrupt options as you go along, like killing a prisoner instead of pardoning them or stealing items that you could just as easily purchase.
Teleport to the Howling Halls in Westcliff and follow the path. You can either defeat the enemies that you encounter along the way or run away from them. Keep following the path until you see a torch.
Turn right when you see the torch and go straight ahead until you see the Demon Door. Walk up to it and it will open if your corruption level is higher than your purity level.