Xerxzes gets up much quicker than any marine has a right to, chainsword still grasped in his left hand. The Gaius-thing is upon him immediately, wielding the force sword of Gaius. A rain of blows are exchanged, so fast that no mortal man could have kept pace. The blades lock, and in that instant the second chaos-blade so thoughtless enchanted by Nouri leaps up and past Xerzes' guard, burrowing through the weakened abdominal plates. Xerxzes falls back a pace, to find himself grasped from behind by Lector Lexander. Seemingly unaffected by his crippling injuries he locks the scion of Fulgrim in place. The Gaius-thing severs Xerxzes sword-arm at the wrist.
Meanwhile the squats have taken to arms, wielding tools and whatever makeshift weapons they have been able to procure. It's not much, but added to the psychic fear conjured by the Lector Locum and the remaining Relictors it is enough to stem the tide.
"Enough," screams the Gaius-thing in a voice not at all like Gaius'. When a small group of fighters do not heed its command quickly enough it sends them flying - allies and enemies alike - with great force at the merest flick of a wrist. "Defy me at your own peril," it shouts with a voice that reverberates throughout the ship.
Xerxzes tries to tear free from the Lector, but the Relictor captain has found preternatural strength somewhere and Xerxzes is going nowhere!
Instead Gaius tears the embedded chainsword free without ever touching the hilt. Next an unseen force breaks the Emperor's Child's helmet seal and lifts it clear of the marine's head. The face inside is twisted by rage, pain, and defiance. Xerxzes' first act is to spit Gaius in the face; the skin on his left cheek immediately turns bright red, then it starts oozing and running as the bio-acid in the marine's spit does its thing. Gaius doesn't even twitch.
"You and me," Gaius says in that strange voice, "are going to be one again."
"Never, " Xerxzes replies curtly.
Gaius smiles in entirely the wrong way, as if his face doesn't really know which muscles to move when smiling. The effect is...disconcerting.
Gaius turns to face Nouri and his thralls. "And what of you sorcerer? Will you serve a worthy master or do you wish to be consumed, body and soul along with your thralls?"
This is where Nouri must decide what to do. Defy what is obviously not some ancestor spirits, but a potent warp entity or abject himself before the daemon?
Lexander has accepted the gift of chaos and is currently a thrall to whatever the Tower is. He could try to fight it of course, but he has already let it in, so it might not be that easy.
Gaius is possessed by the daemon and can't do squat (pun intended).