Obliterators
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These Big Motherfuckers are Chaos Space Marines Who have been infected With the "Obliterator Virus" (Oh so subtle GW...Oh so subtle) Which basically turns them into a giant ass walking Daemonic Engine of Mass Rape It's a large walking Blob of Termie armour, Daemonically Enhanced Killing machine, And Gun...BIG gun.
Obliterators are rare (Usually) And Very Sought after, due to their complete ability to rape anything within a 5 Mile radius. They don't work for any partictular legion or worship any Chaos God individually, but they have their own insider cult with the pursuit of knowledge, alchemy and the fusion of flesh and Stehl. Usually having a connection to techmarines or techpriests they get some KICKASS powers along with the virus. Any object they hold can get merged into them, allowing them to copy them AND produce ammunition for them....Tyranid Digestion Pool?! (Which is kind of fucking creepy when you didn't know you were infected) The metal they get infused with turns them durable as fuck, (Wolverine anyone?) although very slow, but they have teleporters so it's all good. The Iron Warriors have a shit ton of these guys just waiting for commands due to some connection with the Obliterator cult.