Even though this world revered the experts, one needed the support of the local nobles if they wanted to dominate the world. Without having a suitable identity, the nobles would not follow a person. Saleen had been vexing over this matter, but with Lex's help, all these troubles would be solved inherently.
Saleen also knew that Lex had said such words in front his close subordinates to convince them, and both parties would stand a chance of enjoying long term cooperation.
If a duchy was really established, then both parties, regardless of rank, would be bound together. If the fraud was exposed, no one would stand to gain anything. Saleen would lose his influence over the nobles, while Lex's reputation would be worthless.
