Oz, The Great and Powerful's plan to create a new breed of super soldiers to win her war for her.
She periodically captures COG soldiers in engineered "freak accidents", using The Surgeon to turn them into super soldiers of some variety. The majority of her spartans are axioms, with a few being Neurodonors in the form of hell hounds or war horses.
Sometimes the men are brought straight back to her, other times they are returned to their camp to sow dissent and discord before they're eventually recaptured by Oz.
All of these spartans have had pivotal impacts on the war, but they weren't always in Oz's favor.