Long one...
Possibly the most evil thing I did was with Gareth, the paladin in my current Eberron campaign, and played out over the first 45 sessions and over 1.5 years of the campaign.
When creating the background for the character, the player described his PC gaining his paladin powers as follows: His father Byron (a military commander and paladin of the Silver Flame) and he were in a battle where the enemy summoned a powerful demon. It struck down Gareth, and Byron raised his ancestral sword, Kizmet, causing it to shine with a silver light and banish the demon. The effort was fatal for Byron, but before dying he asked the Silver Flame to heal his son and pass his powers on to him. Tira Miron, the 'creator' of the Silver Flame appeared, and complied with the prayer. Byron handed his sword to his son and died.
When the player came up with this, I mentioned to him that Tira Miron (who was subsumed within the actual Silver Flame) doesn't show up to people in Eberron, so it's quite possible that Gareth had imagined that and other aspects of the scene due to being disoriented and on the point of death. He said that was fine.
As the game continued, Gareth found his sword growing in power over time, learning how to communicate with him empathetically. He also had dreams where the Silver Flame sometimes warned him to be wary of his allies. He discovered that the sword's genealogy was something nobody was sure of. He finds that he is stronger with the sword in hand and that it has the ability to weaken outsiders. At one point, he attempted to use it on a daelkyr (an incredibly powerful outsider) and managed to weaken it slightly, but the creature throws off the effect, saying, "No fiend can claim me." All of the above served to significantly confuse Gareth about exactly what the sword was and even had him wondering if he had been affected by a fiend in some way. The party alienist, who had significant issues with the paladin, argued that maybe the daelkyr had been referring to the sword and not Gareth, but he ignored that.
Eventually, the PCs find themselves chasing a rakshasa BBEG who they discover has been manipulating their actions and choices for years, if not decades. Eventually, they bring him to bay at a point where he is trying to free a rakshasa rajah. During the battle, Kizmet speaks directly to Gareth, asking him to help the rakshasa, arguing that this will truly serve the Silver Flame. Confused, Gareth refuses. So the rakshasa looks at him, sighs and calls forth the demon.
Which, of course, had not been banished years ago but had simply been sucked into the sword. It erupts from the sword, shattering it. And Gareth stops being a paladin, since he had never been a paladin in the first place, but only had those abilities gifted to him by the demon in the sword.
The look on the player's face - and all of the other players' too - as everything in the previous 44 sessions of the campaign suddenly clicked into place was beautiful.
