The Warlord I Forged is Secretly Slaughtering My Enemies

I didn't ask to wake up glued to the ceiling of a crumbling gravity-tomb, nor did I ask to be trapped in the body of Tavira Varnak, the doomed villainess of 'The Iron Heavens'. Turns out, the real Tavira magically body-swapped herself with a dying peasant - me - just to escape her execution. I woke up in her body, inheriting her death sentence. If the Varnak clan thinks I'll quietly take the fall for their treason, they're dead wrong. My survival plan? Break Karnal, the royal family's abused, cursed ash-slave, out of his chains and forge him into a living weapon. It worked perfectly. A little too perfectly. After a five-year time skip spent surviving the underground fighting pits, my bruised pupil has returned as the towering, blood-soaked Sovereign of the Wastes. He annihilated the royals and terrified the original heroine so badly she fled the continent. Karnal still smiles at my sarcastic complaints, but I've noticed a disturbing trend: anyone who insults me tends to spontaneously lose their head by midnight. He was supposed to be my temporary sword, so why is he locking all the doors?