During exploration of a recently discovered Space Hulk, the sinister machinations of a cult onboard drag the ship back into the warp with the Slann and his honor gaurd still onboard.
One year later, thanks to the power of unity all Slann hold and some not insignificant effort, the rest of the Slann rip that sucker back into the Material Realm.
Inside, One Thousand years have passed. The Slann and it's Honor Gaurd have spent the majority of that time traversing the massive hulk, slaughtering daemons and discovering methods to attempt an escape from there prison. Along the way, the party slaughters Orks, Tyranids, Hrud, a hellish variety of other Xenos, the cultists and there Master, a literally endless tide of daemons, and finally a possessed Knarloc and it's brood of possessed Kroot war beasts.
They were in the process of attempting to jury rig the numerous ships jump drives to all activate at the same time when the Collective managed to pull them out.
At which point, the Slann aboard ship asked for a resupply, and then asked permission to continue his explorations and experiments aboard the ship. It was a perfect place to examine all kinds of unknown creatures and see how they fought.