Lucifon is the Prince of darkness. Olive meets him in the Western Desert. He led the army against the kingdom years ago, and retreated after his defeat at the hands of the hero.
In the Western Desert
If Olive camps in the place marked "Demon Abode", then upon waking she will meet Lucifon. He will offer her a drink.
If her faith is 20 and up, she will automatically refuse the drink. He will then offer her some power "in deference to [her] spunk." If you accept his offer he will raise one of the following attributes of your choice:
The stat increase can be anywhere from 1-45 points. However, Olive will lose some faith in exchange (anywhere from 1-10 points). Her sin will also increase by 25 points.
Nothing will happen if you refuse his offer.
Note that if her faith is below 20, she'll automatically cave in and drink the liquor which will increase sin by around 80 points. This is quite useful if you are aiming for one of the dark endings (i.e. Princess of Darkness). You need to do this to marry him.
It is possible to get a marriage ending in which Lucifon proposes to Olive and persuades her to run away with him.
To get this, she must have 300+ Charisma, 200+ Refinement and Sin more than double her Morality. She must also have drunk Lucifon's liquor atleast once.
It is considered to be an incredibly bad marriage, and the gods will blame your sins for having let that happen.
Tips and Tricks
This is surprisingly involved for an ending so bad. You need to reduce Olive's Faith to below 20 to make her accept Lucifon's drink. One convenient way is using his gifts a lot to get it low. Focus on raising her Charisma and Refinement after that, and keep raising Sin by killing monsters or accepting more drinks. If you want to avoid Sinful endings you could raise Morality to 30 and prevent Sin from reaching 500.
- YouTube video of Olive becoming a fortune teller and marrying Lucifon.
- YouTube video of Olive marrying Cube and Lucifon commenting on it.