You receive an anonymous message one day from a neighboring kingdom. It's one you've had your eye on already, what with the king and queen ruling as petty tyrants, up to refusing to acknowledge that their child was a butch princess and not a prince, despite the wonders that fantasy HRT was starting to work on her body. A land needing liberation, even if you already know your true motive.
You can already imagine claiming the butch princess for yourself.
The message invites you into the sender's realm, suggesting a ill-defended route through the castle town and entering the palace via a secret passage. The letter asks that you not harm the princess, nor the kindly court mage giving her the body she desires.
You take the invitation, storming into the realm and catching the palace's defenders by surprise when you emerge through the passage the letter told you of. The kingdom is overthrown in a day, the king missing and the queen in chains.
It's when you're sitting on the throne you've gained that your informant (that's me) reveals herself to you - the butch princess herself, who was furious with her parents' misrule and their expectation that she would inherit their mistakes one day.
I offer myself to you, and my co-conspirator - a slightly older, smaller, fat woman with plenty of milf energy - explains more of the details, that she's the loyal (to me) court mage giving me the hot butch body I always wanted for myself.
Of course, you'd graciously accept both of us into your harem, and my court mage gets to join up with all of yours in giving me the body I deserve, culminating in giving me a nice, fertile butch pussy and womb for you to dump your seed in.
I think I much prefer this life of being another breeding bitch than that old dusty life of a "prince" expected to clean up my parents' mess...
It's always fun when they sell out their family and homeland for a chance to be my harem girl. Don't worry, you'll be properly rewarded, you're going to be the princess you were always meant to be...
If you have any lingering doubts, I'm sure that's going to be forgotten once your court mage helps me slide my cock inside your new pussy. When you feel the head of my bitchbreaker hit your womb, you'll know you made the right decision. And when I put you in a mating press and you hear your mage encourage me to breed your fertile butch cunt, you'll wrap your arms and legs around me and know there's no going back...