Author's Note: Glad to see that the fic went over well. I know the first chapter might not seem very Avatar-ish, but I'll try to fix that later. It intend this to be a full crossover, not just "IoM wipes out yet another alien species". This also means the reasons behind the crossover will be explained, as will Pandora's location. Looks like I'm not the only one who saw the Colonel and thought "Catachan"- I was actually considering putting them in the story proper and still might do so. This chapter sets up the story proper, and is actually rather incomplete, but I decided to put it up first and write the rest later. Thanks for the feedback

+++Thought for the Day: The oathbreaker is the vilest of traitors.+++

Segmentum Tempestus. Subsector Gaius. Unknown System

In the 40th Millennium, space is never truly empty. There are space stations, ships of every size and shape. There are space hulks, supposedly-empty derelicts that all too often prove to be inhabited after all. There are meteors, hurtling blindly through space until they are caught in a gravity field and burn up. There are asteroid fields, endless stretches of space where there are only drifting hunks of rock and the shattered remains of ships fool-hardy enough to venture into them. There are even creatures that ply the void for sustenance, their bulk dwarfing any battlecruiser.

And then there are warp storms. These Warp phenomena defy human explanation. To those with the right eyes, or rather senses, they are whirlpools in the never ending stream that is the warp. They absorb the raw-stuff of the Immaterium then expel it again, cutting off entire stretches of space. Should the storm bleed through into realspace, the luckless victims in its vicinity would be exposed to the Warp untempered. Corruption would all but inevitably follow.

Warp storms have a wide range of causes, ranging from rituals perpetrated by insane cultists to the machinations of the Eldar to random fluctuations in the Warp itself. This one, however, was very different from these comparatively normal events.

This storm was ancient beyond measure. It had raged for millenia, cutting off a massive stretch of space from the rest of the galaxy. It had seen the rise and fall of countless empires and civilizations. It was wreathed by rippling tides of warp-stuff, reaching out to ensnare anything foolhardy enough to approach it.

And slowly, very slowly, it began to die out.

M 998.41. Gothic Class Cruiser Cerulean Purgus. En-route to Warp Anomaly Gaius 13.

The command bridge was quiet. Servitors whirred to themselves as they mindlessly adjusted controls and sent reports, their withered hands dancing blindly over the panels. There was a constant chanting coming from the overhead speakers as the ship's priests recited litanies of protection and thanks. In the center of the bridge, blind-astropaths murmured to themselves and twitched sporatically. A tall, cloaked commissar paced back and forth, laspistol clutched in one hand. At the very front, an elaborate, golden freize covered most of the wall, images of the Emperor and His saints glaring down at the crew members below.

Officer Verac paid no attention to any of this. Instead, he watched the shrouded figure who stood at the very centre of the bridge, murmuring to the captain. He was slight, almost skeletal, and wore plain, black robes that dangled from his frame like a shroud. What little of the man's face was not wrapped in cloth was covered by a thick hood, and a pair of goggles covered his eyes and upper nose, giving it the faint appearance of a skull. The overall effect was akin that of a reaper from some feral world legend.

No member of the Purgus's crew had seen the man expose his face.

There was a hiss, and the pair of ornate doors at the centre of the freize slid open. There was suddenly a smell of blood and rust in the air.

"Enter." a voice rumbled, echoing from the vox-casters built into the mouths of the carvings of the Saints. The shrouded man turned, his robes trailing behind him as he passed through the doorway and into the room beyond. Verac heard a snatch of the tech priest's chanting before the doors slid closed again.

Verac blinked and looked away, his eyes suddenly watering. This man, this outsider, was being allowed into the presence of the Captain? This was highly irregular. Normally, only a handful of the Purgus's officers and the tech priests who maintained the command throne's systems were allowed into the bridge's inner sanctum. The Captain was a recluse, preferring to interact with his subordinates over vox-links and through the ship's systems to seeing them in person. Verac himself had only seen the Captain once, three years ago. For the shrouded man to overrule the Captain and force a meeting...

Verac shook off the thought. Whoever this man was, he had authority. That was all he needed to know. Verac had an unpleasant feeling that he really didn't want to know more.

Then an alarm blared, and everything else was shoved to the back of his mind.

Cerulan Purgus. Deck Three, Sublevel Four.

"Attention, all crew members. Warp transition in progress. Glorius Imperator. Attention, all crew members. Warp-"

Commissar Adrian Queris shut out the vox-caster's repeated transmission with an effort, tightening his grip on the laspistol he carried in his right hand. Queris was a Naval Commissar, one of the four assigned to the Purgus, and had served on the massive battleship for ten years. In that time, he had seen hundreds battles and boarding actions. He had rallied the crew members of the Purgus more times than he could count. Executed more of them too. Still, even after all these years, he still found transition into and out of the Warp nerve-racking. To tear a hole in reality itself, to know that your life and soul depended on a handful of cogboys and a Warp-touched...it was unnerving.

A whimper from directly in front of him brought Queris back to his senses.

"Silence, filth." he snarled, clicking off the safety on his sidearm. The ragged figure at his feet moaned in response, clutching its head as it twitched sporatically. Abruptly, it started to mutter to itself.

"A hole, a space, a gap...a tear. Something wrong, something that shouldn't be..."

It broke off and started to whimper as Queris advanced.

The Commissar looked down at the sanctioned psyker, his scarred features twisted into a mask of contempt.

"I told you to be silent, warp-touched."

Queris kicked the psyker again and again, viciously, and the psyker's cries turned into shrieks. Finally, the Commissar stopped, his leggings and boots speckled with red. The sanctionite was curled into a ball at his feet, groaning quietly.

Queris stared down at the quivering bundle at his feet, las-pistol held loosely in one hand. Sirens were blaring in the corridor, but he ignored them.

He'd just opened his mouth to speak when the transition began.

The Cerulan Purgus fell. Coruscating arcs of energy flew past the vessel's jutting nose, swirling around and around the great warship as it dropped through the gap between dimensions. Every living thing on the vessel suddenly felt a sense of nausea and unease as the Warp flared outside. Then, as suddenly as it had started, the Purgus stopped. It was no longer surrounded by swirling warp-stuff, but by an endless void. Directly below it and the two troopships it was escorting, a blue-green globe hung serenely.

For the first time in twenty thousand years, humans had arrived in the Pandora system.

???

The Barrier has fallen.

They are here again. A distortion in the atmosphere, dots of light seen through the eyes of the People as they look up at the night sky. Life that is not life. The Not-People. They approach in their vessels, their tombs of iron filled with life but lifeless. The Not-People can only have one purpose. They are here to plunder and pillage and destroy, to tear down the forests and the plains for their rocks. They are Other, and they would destroy all.

Unacceptable.

Escape is not an option. The Other-Place is tainted now, distorted and impassable. There can be no escape.

There is only one option.

Command Bridge, Cerulean Purgus.

Inquisitor Viel was the first to feel it. As he straightened, repressing a reflexive shudder caused by the transition from the Immaterium, he seemed to hear a buzzing noise from all around him. Shutting out the meaningless blabbering of the skeleton on the throne in front of him, he closed his eyes and tried to find its source. It wasn't as though the half-mechanical fool would be able to notice anything different about him, with his face concealed by the mask. He always wore the mask now, ever since Tempus III.

He felt the universe around him. A handful of sparks in close proximity, one feeble and absent, the others curiously twisted. The Captain and his tech-priests. Small minded, of no consequence, and not the source of the disturbance. Viel stretched out further. More sparks, of every hue and shade. Here and there, bright beacons. The ship's void-born psykers, the Navigator, the astropaths, and Viel's own retinue. These bright lights, too, were meaningless distractions.

Then Viel reached beyond the boundries of the ship, and he found his source.

Light, light beyond measure. Blinding, burning, probing.

Viel realized that he was yelling. There was a smell of burning cloth in the air as the wards tattooed into his skin activated, searing his robe. With a mental effort, he pushed, shutting out his awareness of the universe around him. He hadn't been this close to losing control since his first encounter with the Tyranid swarms decades before. Suddenly, the light vanished, allowing Viel to collapse. He heard yelling and the sound of boots, but it seemed of little importance.

The Tarot had been right. There was something worth the effort here, down on Gaius 13.