The origins of most of the Dukes are shrouded in mystery, legend, and not a little hyperbole. Losik is the only Duke, however, who is known to have risen from the ranks of regular soldiers. A legendary Legionnaire of Doom known for his temper and hands-on approach to massacring people, Losik seized every opportunity he could to amass power. He earned promotion to Legion commander by bisecting his predecessor; he spent all night practicing slaying the innocent, fueled by dark sorceries he had gleaned and some very strong coffee. He was in the front line of every battle, the first to charge and the last to stop hacking when the fighting was over. (Sometimes hours after it was over. Losik was thorough.)
Unbridled ambition among underlings is entertaining for the Dukes, and even encouraged when things are slow; but normally the Dukes would have made sure Losik had a fatal cobra-related accident long before he grew sufficiently in power to be truly dangerous.
The Overlord, however, found Losik useful as a distraction for the other four malcontent Dukes, and quietly protected the fiercely ambitious soldier from the most serious attempts to sabotage him. (He let Losik deal with simple stuff like random bags of tarantulas and explosive bloodwine. That sort of thing just builds character.) Eventually, the Overlord invested Losik with sufficient dark power to hold a Dukedom, granting him a godforsaken territory that had the benefit of "coincidentally" bordering all of the other Dukedoms. Losik accepted his newfound power with homicidal glee and the Overlord watched in amusement as the Dukes spent a few decades sorting out a new balance of power. It was just like reality TV, but a little less evil.