> He had sent a minister on a voyage far over the eastern seas in search of a mythical potion of eternal life. But that expedition never returned

Makes me wonder about the details of that arrangement. What promise could possibly make the minister return from such an expedition?

If he does find the secret to eternal life, the obvious choice for the minister would be take it himself. The emperor would have a strong motive to get rid of anyone else knowing the secret after he got it from the minister.

If he doesn't, well, why go back to a tyrant and face potential punishment for failure?