The Demon Door seems to be of a darker quality than the other Demon Doors. Thus, its opening requirements are that the Hero must be 50% or more corrupt. This can be difficult for the Hero that has chosen righteousness.
On the other side of the portal is a large cavern with defensive ramparts over the portal itself. A narrow passage at the other end leads to a stone bridge over a narrow canyon populated by pine trees. The next room is circular with a slightly raised stone platform in the centre bearing the guild seal. On the seal sits the chest containing The Calavera, a legendary mace. Around the stone circle are a number of short, slender trees, stone walls and doors that, when approached, become highlighted purple but cannot be accessed.
Buy and use cosmetic potions from Knothole Island, as they lower your purity.
Meat and alcohol products have the same effect, but the amount of corruption gained is quite small.
An easy and simple way to increase your corruption is to simply go to an area with shops/homes (Bowerstone Market is best) that you own or are renting out and increase rent to maximum, this can be +100% rent for homes and +60% for businesses, and just wait 30 minutes. Your corruption will shoot up to at least 75%. Lowering prices will reverse the process and make you more pure.
Sleeping in a home which adds corruption also helps.