The carapaces of even the base Xenos were remarkably resistant to laser fire, both reflecting and ablating much the most common weapon of the human Planetary Guards that were their most dangerous opponents.
Sun Shots empowering hot plasma, heavy neutrons, slugs, lightning charges, force lances, and sonic waves, not so much.
While there were suborned PG regiments, they'd been identified and were the first things eliminated. The Mech and Cavalry units had managed to remain fairly clean, as had most of the air cover. The skies were still dangerous places to be with the immense amount of air defenses unrolled from fallen bloks and the enemy heavy units that had breached the Shield Wall... and citizens were not excited to find out that the curious creatures from outside the Walls that made for fantastic documentaries and fight shows were not at all friendly in person, either.
Estimates were that at least a hundred million citizens had been virus-pslaved, and were united and under fairly centralized control. Suborned citizens had gotten in everywhere, and sabotage was ongoing and devastating.
Naturally, there were dozens of unwanted factions taking advantage of the chaos to pull shit, from nobles administering comeuppance to thieves and looters stealing everything not nailed down... to dozens of Warp cultists here to greet the end times and spectacularly make everything go as wrong as possible. Ambushes or explosions in the middle of a successful defense turning it into a disaster happened more than once, and the first few days of the city were horrible. More than a few bloks that had been sealed against the invasion and incursion were betrayed from within, opened from within, or those within simply slain like offerings. A day later, new xenosyms were boiling out of such bloks in massive numbers to add to the fighting.
In certain areas, the hellacious fighting and psychic discharges had attracted Warp energies, and uncontrolled rifts blew open. The shrieking hordes of the Warp came howling out for the blood of anything and everything. Happily, they killed the xenos as readily as anything else, but their taste for civilians was every bit as rabid as the aliens. Slaughter was on the menu, and they were determined to make it a buffet.
Hundred of square miles of Janus Prime were buried in blood and gore, all of which was happily sucked up by harvesting xenos to turn into more syms...
At the same time, there were pockets of fighting that were establishing solid defenses and only getting stronger.
Of the hundreds of Spires that dotted the city, only three had completely fallen, while combat was raging in about four dozen others, and the rest were simply defying the pressure from below with multiple layered defenses, Sealed against anyone and anything from outside. The fact that the Xenos couldn't mobilize any aerial attack force that could really threaten the Spires had a lot to do with it, along with the presence of Mentats who weren't murdered in time to prevent them from being useful.
The Pstatic Towers screeching their telepathic interference into the ether also hampered the enemy. Going from a functional hivemind to needing relays to work together, and that interference only getting worse if an active psion was contributing to it, turned an instantly responsive and agile attacking force into a slowed, lagging army, not much better than their human opponents.
The human forces in Janus Prime seemed to organize who was trustworthy and who was not remarkably quickly, and recognize areas lost and safe with astonishing speed. These areas quickly became bastions for the Planetary Guard to land in and operate from.
The xenos were not expecting the sheer level of violent resistance that came from Underspire, an area that they were sure would be theirs with their numbers and natural advantages. The Skraelings and Themis led a screaming resistance of the Undermobs that often involved collapsing huge areas downspire, burying the xenos and the dead they wanted to salvage, flooding areas with acids, poison gas, and flames, and if cost them thousands of civilians to win, then those civilians died. As long as they weren't recycled, it was a win.
The Soylent processing centers, in the face of so much death, underwent massive necroic events, and hundreds of thousands of animated cyber-zombies clambered up to join in the fun. The Steiners vacated the areas as the xenos streamed in to harvest the biomass, and found themselves tangling with the undead who didn't care exactly what kind of the living they got to kill. Massively poisoned tanks and feeds greeted all attempts by the xenos to get at the biomass, and tens of thousands of vatted Family members were alternately spawned early and thrown into the fight, or killed and turned into toxic sludge that couldn't be harvested as the heavily assaulted vat areas were blown apart with lethal enthusiasm.
The xenos and cerevores very specifically did not expect assaults to be coming down from the surface into their supply centers and bases of operation.
Glowing swords, huge men in armor, death from shadows, and flashing Cloaks. These things flickered across the collective racial memory of the Xenos, and what might pass for dread slowly began to trickle through the collective. In dark and narrow places where unlimited numbers counted for very little, untiring lights rose and charged forwards, ignoring psychic powers, rivers of acidic blood, and screaming black-carapaced horrors as they reduced existential nightmares from before humanity rose from reptiles to a single thing.
Karma.
Watching a wave of cloaked, howling Coronal Guards going over a barricade into the teeth of tens of thousands of pslaves and rivers of xenos, waving glowing Swords and Hammers like shining waves of destruction, mindclaws ripping, Shields humming and taking those charges like exploding walls, did impossible wonders for morale. The fighting power of the Coronal Guards and the units attached to them was so murderous the whole invasion force started to pivot to try to compensate for them... exactly as intended, drawing attention away from more vulnerable areas.
Then there was the Green and Gold pushing back against them, the Rantha Corporate Zone turning into one of the central deployment areas for the Planetary Guard to deploy from.
Just as there were sections of the city completely lost to the invasion, others were hardly touched, and became bases for hastily-raised defenses and sources of manpower for the Juris-led militia, gangs who suddenly found many, many ex-gangers coming out of the woodwork to organize a resistance, and tens of thousands of Planetary Guard moving into attack.
Down in the dark spaces, Umbral Striker teams were slagging key tunnels and transit points, squeezing the movement options of the Xenos swarm tighter and tighter, making their infiltrations of new areas of the city more and more difficult. This continued deeper and deeper in relentless fashion, and the tentacles that had been extended all over the Undercity began to cut off one by one in methodical, coordinated, and lethal fashion, glowing claws and shadows the last things thousands of syms saw before they died.
The Mentat resistance was all focused on the Spires, preventing the swarms from getting to the most skilled, valuable, and richest members of the city. They swept ruthlessly for Pslaved and cerevore infiltrators, and swept climbers from the glittering walls with mass attacks, teams contesting against any concentrations of 'vores with psychotechnic displays of unleashed psionic power.
The bombings began in earnest, scores of acres of city streets drowning under each fuel-air bomb, and if the Pslaves couldn't get off the streets, they were blown to ash, while the reinforced bloks weathered the blasts with fair impunity.
The counter-attack began to pick up steam, as the humanity's machines of war began to gather and bite.
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Celestia remembered thinking something about mechs and power armor and tanks and stuff and not wanting to get involved in them. Yep, she did.
She was one of the most sword-oriented of the Ranthas. Her Talent was made for dueling, and she was possibly the most accomplished of the first generation of Rantha sisters with the soulsword. While she could shoot things out of necessity, she preferred not to have to, and had been indulging that with some very explosive and gory melee combat wherever possible.
Alas, she had run out of suitable things to kill, but they seemed to have called in a friend anyways.
Something big, bright, and space-warping psychic weirdness blew a thirty-foot hole in the building behind her by bending all the matter at right angles to itself, then releasing it to rebound catastrophically in all directions. Everything within sixty feet of the impact point was shredded by matter at fractal thickness flying in all directions.
Except her. The stuff hit her Null and lost all the psychically-granted momentum it had, falling lightly to the ground in a patter of random, breaking debris.
See, if it was a plasma charge, the shockwave shrapnel might actually have hurt her. But probably not.
She run-run-runned herself down the length of the hall (okay, it was skating, but it was still REALLY fast to any normal person), a looming shadow on the street outside casting darkness across the hole behind her. And the doorway after that, and the doorway after that…
Ahead of her, a Legionnaire popped out of a doorway, his popgun ready to shoot, skull-pattern visor turning to lock in on her.
Outside, the big thing out there was winding up for a pitch.
"Get down!" she ordered over the screeching roar that was shaking the whole building with the bass of it, her ki making her voice perfectly audible even in the tumult, and she was pretty sure the idiot just blinked at her in disbelief as she dropped into a slide onto her ass towards him.
Metal began to scream and plascrete to cry as the scythe-blade on the biomech's arm came around. The Legionnaire's head snapped around, seeing a literal horizontal wall of psharpened bio-blade shearing through the entire structure of the building, waist-high in the corridor, coming right towards him waaaaay too quickly.
Celestia's feet smashed into the pillars that subbed for his boots, smashing him off his feet and backwards as his balance went completely out, gyros or no, and yep, he had boosted reflexes, and didn't try to keep his balance at all.
He hadn't quite hit the ground as the huge weapon hummed past and over both of them, carrying mind-bending frequencies of psychic static away with it that yowled and gnashed on her Null as it continued on its course.
A half-ton of man and metal hit the ground with a crash. His helmguard smashed right down onto Celestia's gut as she split her legs to avoid the rest of him, grunted at the impact and used the rebound to lift her head back up as the blade cleared them. She slammed her feet down, grabbing the edge of his shoulder pauldron and surging after the blade ripping its way through everything in front of them.
Above them, the building was realizing that a lot of its lower supports had just been severed and torn out of alignment. It was going to come down and investigate this lack of support with prejudice. Loudly. With plascrete and durasteel timbers and stuff, shaking its rebar in anger.
"I need you to blow out the wall!" Celestia stated in no uncertain terms as she dragged a half-ton of Imperial Marine towards the exit like an oversized clankety-clank something-or-other. His popgun came up as his arm rose to grab hers, he half-turned as his weapon hand came up, and he let go a plasma charge at the wall forty yards ahead of them.
They passed by the last of the scars left by that bio-blade, running in the opposite direction she'd come from as the blast from his gun blew a three-foot hole in the wall.
There was a small problem with the floor in front of them having been blown away by the previous blast.
She heaved him into the air and leapt after him. What? He only weighed half a ton, perfectly doable.
The rockets on his back roared out as his anti-grav kicked in and gyros automatically balanced him out. Momentum carried him forwards. He took one quick look at the back of the biomech outside and kept going straight away from it, setting down just as Celestia's jump hit the ground next to him, and then was startled to find himself pounding after her as the building shook and some upper floors descended down behind us, ripping the wound wider as they did so.