Speaking of drones, there's one group of imperials that has taken notice of them. They might be easy to overlook if you're not used to looking up, but the wyvern corps has extensive training to fight in three dimensions, and they're paying a lot of attention to their surroundings; it may not have taken too long to figure out what the deal was with rockets, but coming under fire by absurdly deadly high speed explosive projectiles that blend in with the sky and outrun the sound of their arrival has a way of making you paranoid. Even once they got the hang of mitigating things (in particular that the rockets are essentially incapable of cornering at speed) and they started showing up more infrequently, its incredibly exhausting to stay on edge for hours on end. It's done almost than the actual losses to their numbers to whittle down their combat effectiveness, especially if you also consider all the time they spend avoiding being killed instead of destroying enemy formations.

It's enough to make them murderous, even by the standards of a group that's known for dispensing violence at the drop of the hat, and has gotten a lot of drones destroyed as collateral damage even in addition to all the ones that get destroyed completely purposefully in the hopes of blinding a person behind the familiar or getting rid of a distraction that interferes with them noticing the attacks. They're not used to being killed hunted like this and are not used to not being able to accomplish things and the intersection is significantly worse than the sum.

That's actually annoying, though they do their best to not leave any sign of it and encourage more destruction thereby. In addition to being expensive and helpful for keeping an eye on the various troop movements, the drones in the air are also very useful when it comes to actually ensuring that long range missile fire ends up where they want it to and not slamming into some completely unrelated building. It's not that they didn't know hitting fast moving aerial targets from dozens of miles away in their own urban environments would be difficult, but they hadn't ever expected to actually need to do it; there are several reasons why air defense systems will automatically shoot down any planes that get too close to a city without changing course, and it's unfortunate that the portal allowed these creatures to effortlessly bypass them.

The only small mercy is that while the beasts in question are extremely dangerous, they seem to lack the ability of more traditional aircraft to destroy large buildings with blunt force impact. This means they can settle for harassment and a steady stream of destroyed drone platforms while closer ranged (and more accurate) measures are arranged, rather than blowing them out of the sky asap and damn the billions of labor hours and thousands of lives in collateral damage.

There's a change of plans. Message from the legate - we've found the enemy army. He's expecting a big fight to come up soon and he wants you ready to participate. Stop hunting around for isolated detachments, and find somewhere secure to hunker down and catch a breather.

And stop trying to get revenge?

It's not like we've gotten anywhere in particular with it, and if we kill enough enemy soldiers we can at least make their leaders regret it. Besides, are you saying you don't want to take your frustrations out one some actual enemies?

...No, I didn't say that. 

They can, for the most part, do as he requests.

(Some of the riders pick their resting spots poorly, and get punished for it with direct rocket attacks or collapsing the building around them, but they've mostly learned better by now)

At 17:37 local time, 6 hours and 14 minutes after the gate first became visible in downtown Schelling Point, dath ilan's military is ready for another serious push. Sunset won't be for another three hours, but they'd like to get the delicate breakthrough operation finished in daylight since it's going to require a lot of precision. (They're also a bit unclear whether a night attack is to their favor in general; they have night-vision equipment and can control the city's lights, but the moon is going to be bright and low light vision is a likely target for genetic engineering, to say nothing of how visibility favors the side with longer range weapons).

They've been gradually tightening the radius until now, but enemy fortified positions have brought a close to the feasibility of that strategy, and they've made good use of the time to prepare their newly updated strategy. Unlike before, there are no simultaneous assaults all along the perimeter; it would mean stretching their heavy equipment thin, to little actual advantage relative to bypassing them. Instead most of the perimeter forces are instructed to hold the line, though where feasible they are permitted to offer probing assaults in the hopes of hiding that and (ideally) discourage any redeployment to the actual breaches.

There are two main lines of advance, each separated from the other by just under two radians of the perimeter, each targeting a random selection from a list of suspected enemy weak points. Ideally it will be unpredictable in advance which exactly they would choose until it's too late, aided by some similar buildups at other likely locations in case the enemy has better intelligence than heretofore demonstrated, but the plan doesn't count on that and should function acceptably even if foreseen.

Are you sure about that? This might have gone your way before, but this time we're the ones with defensive positions, and while these may not be proper fortresses your buildings are pretty sturdy and we've had time to reinforce them. Plus this time you're the ones trying to advance through a field of missile fire to root us out, and whatever your weird armor is made of won't be enough to make you actually immune to it. 

Not at human scales, no. But they're not limited to those. When the vanguard arrives, it takes the form of armored vehicles with heavy composite armor meant to resist rockets and high-velocity kinetic penetrators, supported by repurposed firefighting helicopters, drones, and a handful of actual military VTOL close air support. The arrows and missiles - even the heavier metal rounds flung by superhuman slingers - bounce off sloped armor to essentially no effect, and the first helicopter opens up with a deluge of water. They're not going to take this without killing people, but there ought to be less irrecoverable heads from this than a proper bombardment.

The helicopters' radar range-finding and camera feeds suddenly fail without warning, simultaneously.

That's alarming! They're entirely separate systems and heavily redundant, that's not easy to pull off even if you know what you're doing. Fortunately the helicopters do have large, reinforced windows for back up line of sight guidance.

Those are down too. It's totally dark, pitch black out there all of a sudden; the helicopters are now lit by solely by internal light sources.

Even stranger. They turn on the floodlights, and attempt to radio a report asking for assistance figuring out what's going on.

No dice on either; there's no visible difference between before and after they hit the button to turn them on, though the light system does at least claim to still be operating properly, and in fact they're not picking up radio chatter either - not from mission control, not from the other helicopters, not even from stations broadcasting from hundreds of miles away and safely distant from whatever is going on in Schelling Point.

What the superheated flash-compressed excrement?

It's not an EMP, that wouldn't explain the darkness or half the other data, and while the helicopter has some resistant shielding against that there's no way everything internal would still be working. What does their gyroscopic autopilot system say about their position? 

It seems to think that they haven't moved since they came to a halt and started hovering when their instruments went out, and were moving as expected prior.

The radio blockage and darkness feel almost like a faraday cage. Did they somehow get trapped in... a spontaneously appearing box? No, then it would be dark but the lights on the helicopter itself would still illuminate things, and also they probably would have crashed, and it would be showing up like crazy on the radar rather than just not at all. A cage of nanobots? Some kind of implausibly light-absorbent material that dath ilan has never heard of, that also absorbs radar echoes? Neither are consistent with any other displayed capabilities other than the portal, and doesn't really leave them less confused, but it's at least at least not immediately disprovable. 

They can't continue the attack run, though, even if whatever is causing this wouldn't be lethal to collide with. The inertial guidance should insure they aren't about to crash into something but it's really really really not designed for this kind of scenario. It's meant to keep you from going off course in long flights, not for precisely navigate between buildings with no sensor data or detailed 3d map because that's actually an insane thing for it to be doing, much less do aerial bombardment totally blind, which is so absurd the designers had quite literally never even considered the problem. If they just hover in (approximate) place, can the rest of Civilization figure out what's going on and solve it? They have plenty of fuel, at least.

The helicopter begins to shudder as its motor whines, and the blade sensors start reporting something interfering with their functionality and dropping their RPM. And... the icing alarm is going off? 

When civilization designs aircraft, they put a lot of effort into ensuring they are safe. Not "unlikely to crash," not "functions fine in atmospheric conditions;" as much as possible, the engineers putting them together did their best to ensure that the only things that would knock an aircraft out of the sky are bad decisions by their pilots, and then tried to make that as difficult to as possible. But while the average purchaser is willing to accept some pretty steep costs chasing diminishing returns in safety, they don't tend to have unlimited budgets either, and they don't, actually, have magic. 

A helicopter of this sort is rated to maintain 90% functionality after an hour of typical atmospheric rime from stratus clouds with water content up to 1 g/m3. It's overkill, for most inclement weather, and enough to make a safe landing for almost anything else.

It's not overkill for hostile action; being overkill for hostile action taken generally is probably outright impossible.


Civilization has plans for what to do in inclement weather, and drills pilots on what to do in icy conditions. There's further drills on what to do in sudden onset conditions, and when their visibility is shit, and even how to recover if you missed the very obvious warnings that it was building up or there was a sensor error. This isn't quite like those, both because it's far more severe and because none of them were about what to do if you started losing control in the middle of a battle over an inhabited city. Fortunately, these aren't ordinary pilots. There are a lot of reasons why someone wouldn't want to participate in this operation despite how critical it seems likely to be, so Emergency Services has offered a truly staggering upfront payment and bonus schema to align everyone's incentives as much as they can. Insofar as time and location permit they've been hard at work ensuring the crews in the air are the absolute best available, including flying them in from the other end of the continent. Despite the time pressure and unprecedented situation, most of them come to the same conclusion within a few seconds of each other. Landing the helicopter is hopeless; they don't have enough precision on where they are relative to the city to know if they can safely descend, can't reasonably expect that to change without knowing what happened, and misjudging it would get them killed from hitting a building on the way down. Staying where they are is also hopeless; at the current rate of icing, it won't be long before they crash anyway.

That leaves two options. They can try to quickly gain altitude so they can fly away without crashing into any buildings, and hope that this is both not an instantly lethal mistake and that leaving the vicinity will fix things. Or they can set the helicopter on autopilot to do that while they abandon the craft, and hope both that the craft doesn't hit anyone anyway and that they survive the landing. 

Those are terrible plans. Surely we can do better than that.

Sure. Just as long as you come up with it in the next few seconds, with no aid from anyone outside the cabin, while also attempting to figure out what's going on.

And also every second spent makes both plans and any replacement less likely to succeed.

There's going to be such an incident report later if they survive, wow. Seven of the helicopters elect to ascend.

The helicopter's residents experience the qualia of upwards acceleration, and hear rotor sounds that would suggest they are ascending, but receive no other evidence of this state of affairs. The darkness does not fade as they rise. The ice buildup... seems to slow down, maybe? But it's still alarmingly fast, and it's becoming a serious impediment to flight. Fortunately there's no real risk of them crashing; if there's one thing dath ilan is particularly good at, it's zero information coordination, and their initial positions give an obvious random seed.

Once they manage to get some distance from the battlefield, the build up of ice first rapidly declines and then stops, and the darkness clears not long afterwards; Radio comes back before visual light, but by less than a second. Of the seven, four are shaken but not seriously encumbered; their buildup was sufficiently slower*, or their reaction speed sufficiently swifter, or their initial position sufficiently more favorable, that the build up of ice is still within emergency tolerances and suffices for them to make it to a proper landing zone. Two more need to make emergency landings as soon as they can see where to touch down and will need significant maintenance to be flight-worthy again - not to mention a few injuries from the hasty touchdown - but are largely intact. The last loses power and crashes with all hands. Their heads mostly aren't in good shape, but emergency responders pull them out of the wreckage quickly enough it might not be hopeless for all of them.


*Though they cannot effectively measure it during the incident or communicate the data to each other, later investigations suggest that some of them iced up noticeably faster than others; while even the slowest is significantly faster than almost any natural conditions, the difference in rate between the slowest and fastest to freeze is at least 20% and possibly as much as twice. Physics simulations suggest this was correlated with the outcomes observed, but not entirely predictive.