The mutant tide before you begins to waver, their numbers now diminishing rapidly. Those more compassionate of brothers would note that each mutant was markedly human in features, and all bore faces echoing an immense pain. The experiments upon them by the Tau clearly showing through as their heretic technology was affixed to the fleshy body of each mutant, the insidious mark of their xenos empire clearly shown upon each piece of damning technology. 

"We fight in the name of the Emperor!" Artenius bellowed out, culling another small swath of mutated humans from about himself with the flame-wreathed edge of his glowing force sword, the light emanating around itself forming a corona of bright blue light in the darkened corridors and hallways. Despite their easy success over the horde of enemies and the fact none of them took any damage, he could not help himself but feel some manner of betrayal that they had just shot or slashed sentience that once worshipped the Emperor as they do now.

Composing himself, Artenius looks to his fellows and simply says the words, "Onwards, to the commander of this vilest of places." He stomped off further, his rudimentary knowledge of the Tau lexicon aiding him as he progressed up to the commander's office, which so happened to also be the situation room for the whole fortress. "If our mark is anywhere, he lurks within here. Be ready brothers, for vigilance, is key amongst areas as such."

With effort, the Iron Hand shoved open the alien door with a dull clang. The interior was brightly lit by a multitude of alien terminals, each casting a sickly green glow into the room. In the other side of the room, stood several huge battlesuits, one of which bearing the colours you all recognise as those reserved for Tau officers. The suit is completely motionless, though, the lights that normally indicate operation are devoid of illumination, and the whole armoured suit simply stands there. This room. It does not feel right on the basest of levels. Even those of you without the psyker senses of the Librarian or the heightened senses of the Sons of Russ know that this place is tainted beyond all repair. Faint whispers can be heard to each of you, promising you all eternal glory and life...but only if you pledge yourselves over to the Ruinous Powers.

"This is a damned place, brothers" The librarian began, his words becoming more impassioned with every passing moment. "It is our duty as Space Marines to wipe out this wretched taint, even if it doesn't fall under our role as the Ordo Xenos chamber militant." He stopped himself for a moment, allowing those few words - which more conservative inquisitors may even consider heresy - to fully sink in. "I propose we purge this place through holy fire, each of you has melta grenades, yes? Use them in this place, purge it in a burst of atomic flame for there is no redemption."