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That’s not it lol. Duke Elergy is just a really selfish guy. In the novel it was directly explained that he and Heinley weren’t friends they were just people with common interest so they decided to be nice to each other and that he also wanted the favor of Soveishu to get what he wanted as well. He’s trying to do that by getting close to Trashta. I don’t think it was explained why he’s going THIS far for her tho
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No, they plan was different, Heinly target was the Empire, but Duke Ergi target was Soviet, that's why they can be friend that benefit each other.
SPOILER
Duke Ergi want revenge on imperial family, because his mother that been sold off as concubine by his father, died in the hand of Soviet mother, they framed and killed her but disguised it as suicide. And Soviet also play a part in it though he didn't realized it.
I dont understand why duke ergy is helping trashta??