In a badly designed system, this would be where you throw an exception. Badly designed by dath ilani systems, that is; if this were a project run by the imperium, it would never have gotten this far without failing catastrophically along lines dath ilan values even if it decided to try and save them in the first place, but if by some miracle it had not, this would be when you flipped a coin between security standing around doing nothing and opening fire with their guns. 

In the actual project, there is no need for either. The security themselves are both qualified for their actual jobs and for exception handling, and because dath ilan believes in actually succeeding at difficult tasks there is a line of exception handling for them anyway. Expert advice and experts are tabulated and come to the conclusion that there is still enough probability mass on this not being intended as an attack on civilization to be worth preserving when weighed against worldlines in which that hypothesis is accurate. With references to the prediction markets, they determine the most value preserving option is to send in a specialized team of first contact sex workers, with additional selection criteria of being skilled at dealing with human bodies, masochism or pain ambivalence, and ability to focus and communicate during periods of high sensory input, to distract the alien and attempt to advance the first contact scenario along dath ilan’s intended lines. Meanwhile, another team with sensory blockers, no headache susceptibility, and good self control are sent to retrieve Jacem and his fellow, for treatment, recovery, and ideally debrief. The expected success rate for this maneuver is hovering at about 70%, based on how the alien seemed to react to Jacem employing their own more limited arsenal and their rate of adapting to different stimulations; conditional on it failing, it is also expected (60%) to allow for a significantly more precise gauging of Yslivianne‘s intentions and plausibly (45%) not seriously cost them in negotiations. Armed security will enter the room with them equipped with non lethal weaponry, to guard against the negative tail of the outcomes bell curve. They have lethal force ready and available, but even if it turns hostile nonlethal is judged more value preserving if feasible.

3 such sex workers is judged to be the ideal tradeoff point between increased odds of success and costs of failure, when accounting for the diminishing marginal utility. They bring with them sex toys, specialized cuddleroom cushions, and a handful of other quality of life advantages dath ilan considers standard.

It’s unlikely to be a new situation for someone to try and distract a daemonette like this. Less likely to be typical is for them to be actually good at it, even beyond the relatively impressive repertoire used by Jacem, nor for them to have equipment worthy of the name. Outside of Slaaneshi and Dark Eldar, it’s quite rare for any society  to even consider this a domain you can try to be societally good at, much less put effort into actually succeeding; among those civilizations that do not demonize sec work, there is a disturbing lack of engineering know-how and effort spent on optimization of the experience. 

In comparison, the team here to retrieve Jacem and his cohort are much easier to miss!

Uhh. These people sure don't know what they want, do they? Are we fighting, or are we being willingly corrupted? Or... do I get to choose?

Obviously, I'm choosing corrupting the willing! I'm not some Khornate brickhead! I'll trade weirdo nerds for "sex workers" any day!

And hey, these "sex workers" are already almost there! Just a slight push here... a light touch there... a few good old Chaos runes... and here we have it! These people are now thoroughly playthings of Slaanesh, as is good and proper! That has to be celebrated!

Yslivianne kisses one of the "sex workers" deeply, and her soul departs into the Dark Prince's Realm, to experience interesting mindstates for the rest of eternity (?). The other two start to enthusiastically tear her apart, with the intention of feasting on her meat, which Yslivianne will also join.

I mean, these three people obviously came here for some good ol' debauchery. They brought torture toys and everything!

It wouldn't make any sense to get upset by a little mutilation now, would it? Hell, no one have even reprimanded me for eating the summoner!

Yeah, no.

Subtlety is abandoned. A magnetic grapple is fired at the sex workers, clipping onto a matching magnet on their back  and dragging them away. The rescue of the original contactees doesn’t have quite the same elegance, but they’re also not surrounded. Both attempts have the potential to cause some injuries, but leaving them there will cause more and also likely other negative effects. Simultaneously, a net covers Yslivianne and then magnetically locks against the floor, before tightening to reduce the room to struggle. They have tazers and gas, but there’s no need to take risks with electricity or potentially do experimental xenobiochemistry on the spot if kinetic force and mechanical engineering will do the trick. The net is made of woven strands of carbon fiber and armored against cutting by a layer of titanium alloy, and are immediately followed up with by further restraints. 

An observer might call the timing and aim of the shots inhuman, but a dath ilan would tell them they could do it too, if they tried hard enough. It’s not like it’s that hard to redevelop the athletic principles and engineering required from a standing start, after all.

The exits to the outside world have been thoroughly sealed for a while now, but once the non security personell are evacuated to a section of the cordoned off area for emergency treatment, that section will seal off too. 

The security are chose for their competance, experience dealing with info hazards, mental discipline, and lack of any headaches or other mental economagic responses. These requirements are rated at dath ilan’s standards, but even then when considering keepers that’s still a much larger recruitment population than you might think.

Even among this group, armed as they are with equipment designed to limit their inflow of sensory data and keeping their distance, they aren’t unaffected by the aura, just able to work through it. It’s this very downside, however, that reveals a useful - if worrying - data point. As long as they are in combat, the strength of the attraction diminishes; not a lot, but noticable, when you track your mental state as much as keepers do. Dath ilani are not idiots and do not uncritically accepting of this as true, much less a positive, but it’s added to their models and as an emergency measure, to be tested when a safer way is found at a future date. The effect is lesser, but still noticeable now that they know what to look for, when they go from direct combat to ready position with an eye out for responses.

“Dath ilan does not appreciate harm to our diplomats, nor mental interference with the execution of our duties. Cease, and this need not get worse for you.”

AHHHHH WHY YOU BROUGHT TORTURE TOYS AND EVERYTHING

Screw it. She've had a good time, she have thoroughly doomed this warp-naive planet, these people are CONFUSING and this net is HOLDING HER and THE SEX IS OVER and you know what?

Her work here is done. Done, she says!

"Yeah, I really don't get you guys. You do whatever. Maybe that Light thing'll be good for ya. Toodles!"

And Yslivianne leaves for Warp, as her body dissolves into an organic mess.

This could have been worse. Nobody says that, of course, even though in dath ilan that particular story trope is far less common, but the thought is there. There is a reason most alien invasion rehearsals aren’t reslistic, and it’s that they’re too depressing. Sure, this was in some regards an almost ideal situation, with only one alien coming into a location already under surveillance by armed keepers, but for some possible aliens - especially economicmagic aliens - that would be enough to destroy dath ilan. That’s not to take away from the tragic loss of at least two and possibly as many as 4 lives, but it was a risk they knew they were taking just by attempting this. Some heads will roll on the markets that’s in retrospect, somewhat underrated the danger, but with the numbers they had to work with the plan was reasonable even in the post mortem.

The victims, once it is determined they are unlikely to make any kind of recovery and have no useful tactical information they are able to convey (in dath ilan with keepers, this is a very swift process indeed, especially considering the state they’re in) they are prepped from onsite cryogenics. They’re significantly less confident than usual that they will be able to wake them up in the future, but unbounded artificial intelligence and economicmagic are both unknown unknowns when it comes to capabilities. It’s not pointless, not like the former person now forming a twisted ball of ex-flesh under the net.

Of course, just because it seems over doesn’t mean they come out of lockdown or the base goes off alert, just that they undergo screening questions to assess the effect of the mental attack and exhaustively record their experiences and impressions for analysis. Others within the cordoned off region set up the instruments for remote examination of the remains of the person, in case there is anything to be learned from it without getting any closer. Everyone who entered the room undergoes a thorough decontamination to cover the relatively long odds there is material cause, and everyone even near it is to undergo thorough surveillance during the waiting period. The reports will undergo infohazard screening by an entirely unrelated group of keepers before a limited dissemination occurs.

Across dath ilan, an advisory of potentially hostile aliens is put out. Dath ilan’s population can be expected to react reliably intelligently to that, as well as details of how to detect their intrusion or mitigate the effects. The specific methodology employed in the effort to reach out that cumulated in this is also banned, covered incidentally in a set of broader rules on economagic experimentation that happens to include this and a few other strategies with plausibly similar results. It’s not that they expect sharing how they did it would lead to a copycat among their population in a less secured area, but why trust what you can cheaply verify? Devices are prototyped with the aim of detecting the physiological changes associated with the mental effect; both false positives and negatives are epxlexted, especially considering their theories on other categories of mental influence, but they will be noticeably better than nothing when used intelligently and centrally tracked. Militarily, dath ilan is already on alert and there are serious costs to increasing readiness too much higher for too long, but preparations are made for responding to potential incursions or arrival via an alternate method of more aliens similar to the one they encountered. Training of citizens in those keeper techniques that proved most effective is prioritized, especially those in the military, governance, exception handling, and logistics. Surveillance of everyone who experienced headaches continues, with a somewhat different list of things to primarily look out for.

Well, with the echoes of the call dispersing relatively far in Warp, and a survivor returning to tell the tale, the Immaterium around dath ilan is soon going to get filled with a lot of daemons of all four kins lusting for it and spying on it and seeping into the dreams and fantasies of people. Eventually, it's location is going to make it to the Chaos Space Marine legions, Chaos cults in the Imperium and Chaos sorcerors, and from there, to who knows where.

Though recognized as weird and curious, the sheer importance of this world is largely lost on the daemons, except for the Greater Daemons of Tzeentch, who, being made of planning, aren't babblemouths.

This about the daemons of Tzeentch: They come in two categories. The first one are the lesser demons - who embody the more direct and physical aspects of Change and Pursuit. Though great at throwing combat magic around and turning things into different things, as well as being utilized as power sources and tools, they are barely stable, obsessed, insane, and rather freaking dumb. Not someone you'd want to trust negotiations to.

The greater daemons and daemon princes of Tzeentch, meanwhile, embody the mental side of Tzeentch, the conscious organization of Change through Knowledge. If the minor demons are insane fools, the greater daemons and daemon princes are insane geniuses. And their insanity is always uncontrollably contagious.

For that is how most of the Galaxy percieves - and uses - knowledge.

Which, of course, means that the greater Tzeentchian deamons are also terrible diplomats when sent towards unprepared people.

(And Tzeentch himself is a perfect diplomat, of course, but he can't just go manifest in realspace willy-nilly!)

Usually, genuine negotiations of his faction with unprepared people would be conducted by his sorcerors.

What a lucky, lucky coincidence it was, that there weren't any Tzeentchian daemons around during the first contact. The God of Planning cannot just go ahead and leave such a terrible first impression on those wonderful, wonderful newcomers.

And if the forces of Slaanesh, the God second most aligned to dath ilan, get to embarass themselves... well that's just so unfortunate.

There may eventually come a moment where the Father of Lies and Deception will need to lay his mind bare before the very clever people of dath ilan. Tzeentch considers erasing this plot from His memory - as would be trivial for the Lord of Change - but decides against it. This wasn't much of a transgression, and He cannot adopt this sort of deception as a policy - those with excellent grasp of probability would piece it together.

He'll be patient. He is patience himself.

And when the time comes, He will welcome His wayward children with open arms...

The information of the new planet seeps to other Gods, and, though they only expend miniscule amounts of attention on it, they have their reactions.

That concept of "ill-advised" really is quite pathetic!

Yslivianne have gracelessly screwed up, and I didn't feel an impulse to be merciful at the moment, so she is no more.

But it seems like the cuties are on the right track, what with their Light doctrine and sex workers and all. And they didn't even destroy my new playthings! Freezing them? That's isn't quite going to do it, dearies.

Come to me, little planet, I have the most QALYs. SO MANY QALYS. YOU KNOW YOU WANT TO.

THIS PLANET MAKES KHORNE DISPLEASED.

THEY ARE.

A BUNCH OF.

WUSSES!!!!!!!!!!!!!

These people are... really trying too... hard. Should... chill. I should send them some... gifts sometime to... calm them down...

Husband, I don't think that’ll work.

These people won't become chill even if all but the last of their kind succumb to your, ah, gifts. I can sense it.

(If Isha wasn't held as a half-prisoner half-refugee, she would back dath ilan so, so much.)

Eh... whatever... 

Some time later...

...and the relic was recently recovered by the Blood Ravens. And that's the interesting thing #1391 since the last time.

Okay... moving on to #1392. A human-populated, non-Imperium planet named dath ilan have appeared in coordinates such-and-such seemingly out of nowhere. Being ignorant of Warp, they have summoned a daemonette and successfully rebuked it. They are REALLY aligned with me, more than almost any Imperial world, impossible to miss when noted. Weird.

Interesting thing #1393... The Cleaved have designed a new bioweapon, specialized for...

Many thanks to you, my dear.

And, you know? With remote destinies shifting wildly, who knows, maybe our cause isn't as hopeless as we have thought! We may yet to have the last laugh! So stay strong, and under no circumstances give up!

Ha-ha. As if you could do a thing directly contrary to yoour portfolio...

Well, I'll be off. There's much to rehearse...

Ever wondered how the Black Library stays ahead on the secret Chaos lore?

Well, one of the reasons is, they have an insider with access to Chaos's highest ranks.

(Isha's husband should really know better than to be such a blabbermouth.)

There are other reasons. For example, one of the titles of the Black Library's master is "the keeper of secrets", lowecase. This title have stayed on a high-ranking Slaaneshi messaging list for literal millenia, and the delivery goes through so many secret intermediaries that no one who matters have managed to connect the dots yet.

Cegorach thinks that that is really, really funny.