As the game really lends itself to a story-like narative, I'm going to show you what I've written up about my adventures. There were five of us playing, but I couldn't keep track of what everyone else was doing, so this story has to come just from my own vantage point.
Hera
Even behind thick steel walls he could hear its heart beating in a sickly, errant rhythm.
It began on the hills near the complex where she was being held; deep in the administratum, in the powerful fist of an energy containment field through which nothing could either enter nor leave. The mysterious Ranger from Shad'el-Khaine covered Baastan as he moved forward through the ruins; debris of an Eldar city (ancient by even their standards) in which the Imperium had erected a science facility like a vile stain marring an aging work of art. As Baastan silently stepped from ruin to ruin, advancing on the administratum, the Ranger watched for enemy movement and noticed a peculiar sight: A single Tau soldier, standing well back from the Imperial building and its guards, stepped out from behind the shadows. Perhaps he was there on a mission of peace, perhaps commerce, perhaps something more sinister... but his presence here was odd. Baastan spoke a message to him through telepathic signal: "Carry out your purpose here on Hera as you please, for we will not seek to obstruct your delay your intentions. Be aware, however, that you come at a dangerous moment and great evil is here that must be subdued. Do not prevent us from fulfilling out mission to disperse the Daemonic energies and we shall each be able to leave this place alive." The Tau, surprised but not without his nerve, agreed and communicated to his fellows (two humans inside the adminstratum who were carrying out a commercial transaction). Alas, it was too late.
Baastan began the raid on the building by detonating an Eldar blind grenade to neutralize the offensive power of the guards. As they found themselves stunned and in disarray, the Shad'el-Khaine sniper rifle began to fire over Baastan's shoulders (bringing his opponents to the ground) as he clung to the building wall, edging his way forward. As he came around the corner to the front face of the administratum, Bastaan unleashed a storm of shurikens from his weapon and neutralized the Guardsmen who would seek to block his access to the Daemonic quarry. Apparently the human commercial effort was not going particularly well, as gunshots rang out from within the building and one of the Imperial soldiers was blown straight through the wall in front of the Eldar assassin's eyes from the force of a combat shotgun.
Now, with the confusion of combat and the scrambling of Imperial Guardsmen about, time became more of a factor than ever. Baastan could see two enemies rushing to the hatch of a Chimera tank in an effort to both protect themselves and to bring heavy firepower to the fray. This would most certainly have proven lethal to the Eldar effort, and Baastan moved quickly. Quickly leaping to the front door of the administratum in one movement, he dispatched the two remaining Guardsmen (both wounded) with shurikens to the back of the skull while throwing his one Haywire grenade at the vehicle. A dull cracking sound split the air as it detonated, and all electronic equipment in the area suddenly failed. The Chimera was dead and useless. Lasguns were rendered inert and still. A keen advantage might have been gained and, in fact, savored by Baastan if not for the one mistake he did not foresee: He had neutralized the electronic energy containment device within the building which held the Daemonhost prisoner. He had unwittingly released the true enemy. In an explosion of sickly green light that blew the third floor out from the adminstratum, the Daemon came to life, and now every living thing in the area was in terrible danger from her wrath; for daemons do not care to be restrained.
Baastan could not afford the luxury of reeling in awe at the power of it; he had dodged the grenades and gunfire of the still-living Guardsmen and found himself quickly leaping forward, sword drawn, to assail one soldier who was clearly not going to afford him any respite to deal with the mission. After a quick locking of swords, the Guardsman dove quickly away to seek cover both from the swift Eldar blade and the encroaching daemon, thus opening an opportunity for Baastan to displace and find a quick escape.
Already the daemonic energy was rife in the area; an electrical charge was in the air making Baastan's hair rise on his head, as if a great lightning blast was gathering only an inch away from him. Humans ran in every direction and the sound of gunfire was ringing out. He might have noticed that his partner, the emissary of Shad'el-Khaine, had rushed forward in an attempt to place a detonation charge on the building itself (in an effort to destroy both the adminstratum and the foul beast within), if not for the bizarre and unnatural sight before him: Two dead Guardsmen, eyes as lifeless as stones, climbed to their feet. As blood poured from their open wounds and their heads rolled back and forth on their necks, Baastan could see that they were being controlled like flesh marionettes by the Daemon's will, and as they advanced on him it became clear that she knew not only that the Eldar were here... but what they were here for.
Trying to protect himself as best he could, Baastan took several agile steps back into the rubble and swung his blade back and forth against the advancing puppets. It was then that the building erupted in a violent blast of fire and brick, as the Shad'el-Khaine sniper triggered the detonator from a distance with a bullet. Now, as Baastan remained locked in melee with the two dead men, he could see her twisting, pulsing visage. She was horrible to behold and her odor made him wince. There was a strange river of harsh chatter and shrieking in his ears and he felt her eyes boring into him with hatred and malice. He realized that whether he succeeded or failed in his mission, its end was only seconds away. Though humans on the ground around him fired round after round of gunfire at the Daemon, he was still pursued by her puppet dead who advanced clumsily but deliberately.
Baastan knew this was the time where his responsibilities had to be discharged. Whatever the Seers of the Craftworld divined about the appearance of this abomination, this hand of Slaanesh, he knew that great harm would, in one way or another, come to the Eldar if she were allowed to survive. He also knew that it now rested on his shoulders to end this.
He refocused his gaze on the dead Guardsmen who pursued him. With one slash of his Eldar blade, the first lost his entire arm. With a second, the second lost a leg and fell to the earth. Though they grabbed and struck at Baastan from their weakened positions, he had already lept away from them, climbing the rubble with preternatural speed and advancing on the daemon. Gun blasts kept her attention distracted from him but, otherwise, he would have easily had her full attention. The Eldar are beacons to the Daemons of Slaanesh; even among a million humans, the souls of just one Eldar can be discerned like a sharp beam of light. The mortal host of this particular daemon, however, was crumpling under sustained fire from the humans who advanced on her, but Baastan knew that would not be enough. He lept forth and, with a prayer of Eldanesh on his lips, he severed her cursed head from her shoulders. Her human host had been destroyed and all around them could feel the Daemon trying to claw its way back to the material realm, but to no avail. As the corpse hit the ground and the air cleared of the Daemon's energy, it was all over.
When the humans regained their wits only moments later, the Eldar were gone.
