Give them a choice between being a protectorate and being assimilated.
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You make a quick decision. These Xenos would be better off as a protectorate, both for your treasury and for the future well-being of mankind, but if you could yet seize their loyalty you would have an entire, if inhuman, pseudo-civilized world at your disposal. They would require relatively little effort to uplift and might function as an adequate force to draw auxiliaries from, depending on their nature, possibly even on par with the units of Antwir. You seize the opportunity.
"Our terms are that they must choose, to be a protectorate beneath our banner or to be assimilated, and march in the name of the Chapter." The Prognosticator pauses and replies.
"You're giving the Xenos a choice, Brother Talnior?" Your voice deepens.
"Do not doubt my dictates, Prognosticator. Where you have the gift of prophecy, I have my reasons." He makes a grunt in affirmation.
"...Very well." The hideous sounds resume, curt and to the point.
When this is finished, the Xenos leaders turn to argue between themselves, even more viciously than before, and slowly stop their bickering to smack and gnash their maws at the Prognosticator. He communicates immediately. Outside of the helmets themselves and the bridge of
The Warmonger, these comms have been completely silent.
"Brother Talnior. They are... in disagreement and request further time to consider our offer. None of them seem opposed to us, only over the manner of their future relations. How should we respond?"You make a snap decision.
"This is a substantial choice to make. We'll give them all of the time that they need. For now.""Bah, let the horrors deliberate for a few more days. It matters not. The Chapter can wait.""These aliens will not waste our time. I expect and will receive a response in the next hour.""The Xenos are to settle this immediately, through an honorable trial by combat."