Hmph. They seem vaguely militarist xenophobic, aren't particularly compatible with our weapons if we wanted to make auxiliaries of them, and likely aren't producing enough resources to make too much of a dent in our needs. Can anyone think of how we're going to profit from being here?
Send back a transmission with the Fibonacci Sequence and an offer of peaceful annexation. And Bolter Rifle fire, to hint at the fact that our guns are better than theirs.
If they've got this much tech they could, if NOTHING else, be enslaved to produce resources. Might be very interesting with their apparent weird means of locomotion.
Set to the work of translating their language as quickly as possible and move the strike cruisers all the way into low orbit to search for weapon systems. Rest of the fleet will hold further out, ready to provide support but out of immediate danger.
Like both of these. Though maybe we keep the Battle Barge back a bit, just in case.
Alright fair, though I doubt they've got anything that'd actually hurt us. Edited the proposed action a bit.
You find these Xenos repulsive and think that their alien physiology may be too incompatible with human munitions to make for proper auxiliaries, assuming you dared to arm these creatures. Their technology is likewise primitive, from what you can tell, and would contribute little to your empire's deficit of resources unless you were to tithe an amount far in excess of what most free human polities would endure. Even so, if they are dangerous by some means they may yet be serviceable where human or Astartes lives would otherwise be spent, and every bit of industrial input, no matter how small, is indispensable. You aren't sure when the Kroozade will return. You drove back an offshoot of theirs decades ago, at great cost in auxiliaries, and know well that they consider you 'eretikz worthy of a good krumpin'. You don't dare to fight them until you have every advantage at your disposal. These Xenos, revolting as they are, may be to your advantage, and so you hold the possibility of orbital bombardment in reserve.
"We will draw closer to the Xenos. The Battle Barge, both Gladii, and the Nova are to be held in reserve, the Cruisers are to enter low orbit and survey for weapons systems. Prepare the voxcaster, unencrypted, down-tuned to radio frequencies their subpar machinery can grasp. I intend to respond to their transmission... in kind. Am I understood?" The officers of the bridge, Astartes and mortal alike, assent.
"Yes, Chapter Master." "Right away Chapter Master!"
"Your will shall be done, Talnior." They move to follow your orders with careful efficiency.
At the same time, a vox-caster of sufficient power to reach their surface is prepared and your message to them, with it.
"ATTENTION, XENOS. I AM CHAPTER MASTER TALNIOR OF THE WATCHTOWER. MY FLEET IS IN YOUR ORBIT AND POSSESSES A POWER THAT YOUR FEEBLE MINDS CANNOT BEGIN TO IMAGINE. KNOW THIS, YOU WILL SUBMIT OR YOU WILL BE SLAIN BY THE MULTITUDES AND SUBDUED BY FORCE. THERE IS NO NEUTRAL GROUND. DEFY ME, AND YOU WILL FIND THAT I HAVE NO MERCY, AND THAT YOU WILL RECEIVE NONE." When you've finished speaking, you hold the Eye of Death into the air and pull the trigger, magdumping directly into the ceiling.
*BRRRRRRRRRRRBLAMBLAMBLAM* The auxiliaries flinch and recoil but the Astartes don't move a muscle. The least of them has been long-inured to the sound. The audio is spliced in after the Fibonacci Sequence, looped, and sent downward at every major frequency you detected, including those they thought were secret. You feel that you may have been a
touch too diplomatic but a measure of leniency may get you somewhere. If they disapprove, none of the other Astartes say anything.
When that's done, you turn your attention to trying to decipher their broadcast, along with the sharpest minds of the fleet, among them the Master of the Forge, multiple Prognosticators, and those Veterans who have the most prior experience in dealing with the Gremlins. The distinction of the bridge, yourself at all-powerful command and they in quiet deference, evaporates into a calm, serious dialogue where each voice carries equivalent weight. You know all too well that your expertise, while extensive, is far from absolute and you take no shame in deferring to those of your brothers who are better informed. Each of you pores over the sequence and audio footage from their radio traffic for the better part of two weeks, each and every smack, snarl, and squelching noise, even their gunfire, to try and wrest some tangible meaning from the noise but at the end find you know no more than when you began. Finally, you scowl and conclude that you'll be getting nowhere without a more hands-on approach.
The sensors report back that they've adjusted their radios to more minor channels to continue broadcasting among themselves. You begrudge them this, so as not to heap undo pressure onto them. They seem to have taken the broad meaning of your looping audio into account, judging by the more hushed and frantic, even frenzied tone of their gibberish but you doubt they are any better informed of the exact translation than you are of theirs. The Cruisers report back that they've found over a dozen large iron domes at key strategic points capable of sliding open, which they suspect may contain primitive atomic missiles. There seems to be no heightened readiness surrounding them in the wake of your appearance but their garrisons and fortifications already represent what would be a tedious obstacle for a squad of Assault Marines, let alone foes at technological parity. These Xenos seem to have a deep philosophy of military preparedness. How much of that is aggressive, you aren't sure. If nothing else, you can conclude they are less naive than most.
Eventually their radio frequencies sputter to a stop and your sensor arrays catch another broadcast launched into orbit, ignoring your Cruisers and directed toward the Battle Barge. It is shorter than the first, punctuated by only a single gunshot, and followed by a series of monotonous pings. The Techmarines grasp the meaning in a matter of hours. These are coordinates to a specific point, outside of one of their more centrally located cities, where marshland has been drained and filled with rockcrete in an attempt to make a landing area sized for the Battle Barge. You can see an enormous gathering of some thousands of half-tracks and tens of thousands of the shelled, armed Xenos on them surrounding it in a loose circle. None of them appear to be civilians, although you can pinpoint a specific group of Xenos who lack weapons but are surrounded by thick knots of their kind, which appear to defer to them. These must be their leaders, then, which means that they're attempting to reach out diplomatically. The sensors can't find any evidence of atomics but you wouldn't put it past the Xenos to have them hidden from orbital view and primed to explode beforehand. After all, that's what you would do in their situation. You strain to understand the best course of action. The Prognosticators say nothing but you agree with their earlier insistence, that directly tapping into the mind of a Xenos would be more efficient than sifting through grainy audio by an order of magnitude.
The Xenos seem to be as prepared for violence as their level of technology conceivably allow, but they haven't attempted to attack your fleet. This may be due to caution of your strength, a misguided sense of proportionate response, or some quirk of their alien psychology, but you suspect returning their approach in kind, presenting force while delivering none, would go a long way to pacifying any rebellious impulses in their population. Of course, this may all be a trap and the best course of action to bombard them from orbit and spend a few years mopping up resistance. The Codex Astartes goes into little detail on the tricks of the Xenos, only to say that under no circumstances can they be trusted. That may be true, but you don't trust the Imperium or the "wisdom" of its dictates either.
How do you want to respond to this apparent outreach?