Transformed into a Demon Queen: From the Start, I Broke the Heroine

[Transformation + Invincible Protagonist + Gender-Bending Romance + Western Fantasy] “Miss Hero, you can either run away on your own or come with me to the Demon King's castle and help protect the world. It's your choice!” Lucia had only just transmigrated into the body of a Demon King when she encountered Miss Vivian, the defeated Hero. She promptly gave her the option of death. “As long as you don't destroy the world, I'll be your little cat. Please... spare the world...” Desperate to save herself, Vivian had already begun saying anything that came to mind. Little did she know that her words would pique the Demon King Lucia's interest. “I thought you only cared about yourself. I didn't expect you to go this far to save the world.” “Miss Hero, try calling me Master first.”