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The problem I see in the original writeup is that Sir Tristen is that his murders were the result of a curse instead of his own volition.  I change it as follows: He doesn't get a domain as soon as he gets taken into Ravenloft but is deposited in Mordent. He hears of the Alchemists Apparatus and finds the records for it. He doesn't have the skills to make it but tries to do so anyways but they all fail miserably. He then  gets a mage to make one under threat of exposing a serious crime the mage has committed. When he tries using it the machine malfunctions, instead of dividing the entire entity into two different bodies it divides his good and evil into two personalities. Sir Tristen mind snaps and thinks it worked completely and that the bad half of his personality is in a separate body going under the name of Malken. He will admit this to no one. There is also no way to cure him of this delusion as it is a strong part of his personality.  Because of the circumstances of Malken committing dastardly crimes shortly after Tristen does something good he is getting paranoid that Malken, or his agents are constantly spying on him.

It's implied in Gazetteer 5 that Malken is the soul of Tristen's father, which is passed along his descendants. So it didn't matter so much what Tristen did because it's Malken that's the darklord. Tristen is just the unfortunate host.