Now here's the twist that made me want to write this fic...
Andrews stood, almost crouching, with one foot in the trapdoor. One arm was around Irene's neck, and a spike on the head of his ax pressed against her throat. But the eyes of Jackson, Zed, Andrews and even Irene were locked on Colby as he rose, muttering, from the floor, clutching what looked like a gash in his scalp from the glancing blow that had knocked him to the floor. "What?" he said. "That jackass gets one in behind my back, and suddenly everyone looks at me like I'm the bad guy?"
He turned to face his companions. Jackson continued to stare, Irene began to sob, and Andrews abruptly cackled. "What?" Colby said, waving his bloody hand at his attacker. "What the f* 's wrong with you?" It was then that Zed got a clear and conclusive look at the small patch where a bit of Colby's skull had been knocked away to expose his brains.
Jackson finally raised a trembling hand to point in accusation. "You!" he said. "It's you! You're a Thing, man!"
"What? What the f* are you talkin' about?" Colby shouted. "I'm no Thing!" As he jabbed the air in his gesticulations, the nail of his pointing finger grew into a claw. "You are the Thing!"
Irene covered her mouth. "Oh my gawwd..." she whimpered.
Jackson, on the other hand, only stepped forward belligerently. "No, you're the Thing!" Then the claw, and the whole hand behind it, plunged into the chest.
Irene screamed, and Colby roared: "You're the Thing!"
Then Jackson struck Colby over the head, with a fist that was suddenly as big as ham and covered in purple scales for some reason. His jaw literally unhinged as he bellowed:"YOU'RE THE THING!" Then there were no more words, only shrieks, screams, roars and less classifiable sounds as blow followed blow from deadly appendages that multiplied like the heads of the fabled hydra. Irene screamed, and screamed, and screamed again, and Andrews laughed and laughed.
Andrews retreated down the trap door, dragging Irene after him. Zed lunged after them, but had to dive to one side as Colby and Jackson went whirling by. Zed rose to a crouch, only to see the trap door slam and lock. Then he jumped up to avoid the combatants as they came reeling back. The myriad limbs were flailing all the more furiously, but no longer withdrew when they struck. Instead, they sunk in, quickly merging with the flesh they penetrated, so that, even as they battled, the two Things became one.
A terror bird's head slashed at Zed's throat. He ducked under the blow and then retreated, circling the center of the base. The chimera came after him with a chorus of screams. Zed ducked out of the room. As he exited the next room, a crash resounded behind him. A glance back showed a single blue mass shaped like two runners in a three-legged race. The faces of Jackson and Colby were still recognizable, one fused to the other on a pseudopod head that hung below the hunched back. Zed halted at a second trap door. He fired one steel ball slug into the lock, then emptied the clip as he backed out the next door. His visor lit briefly on a wired bundle of explosive charges.
The chimera crashed through the wall of the lookout room on eight hands and feet, with still more stretching out behind it like a dragon in a Chinese New Year's parade. Already, the floor was covered in snow and ice, and the polar winds blew even more forcefully through the broken window than they did outside. The blast of cold all but staggered the chimera. Nothing but distorted vestiges remained of the joined faces of Colby and Jackson, except for the sockets that held three flaring red eyes. This visage shrieked as Zed emptied a fire extinguisher into its eyes. From between the eyes, spiked radula lashed out, but snapped back from the spray which added to the chill. Then Zed retreated yet again, with a spool of wire in one hand.
The chimera skirted the still-burning bodies of Thorson and the Indian, accelerating with the added heat, until it was going even faster than before. It was just starting to slow down again when it caught up with Zed, at the room with the second trap door, just as Zed dropped through it. He unrolled wire behind him, and pulled the trap door down after. He held his grip on the bottom, putting his full strength and weight against claws that scrabbled at the edges, and straining even then. With his free hand, he opened a servo housing and eased the end of a wire toward bare circuitry.
The serpentine form of the chimera twisted almost in a knot as pried with ever more formidable implements at the door. A sloth's claw dented and scraped the metal, but without the full length of a sloth's arm behind it, there was not enough force to lift the door. Then the chimera brought its head to bear, with a newly sprouted set of sabertooth jaws. Slamming down its open maw where the claw had already damaged the metal, it broke both fangs, but put one splintered tooth through the door. Pressing its lower incisors against the door for leverage, it began to pull. As its muscular body twisted, one hand pressed against a bundle of thermite charges.
And then there was light.