When the spell ends, you know you did not succeed. The synchronisation wasn't perfect, and you are now the Dark Master's slave.
You can feel what Ebony was feeling: the unbreakable bound with the Dark Master. He is your master, your world, your reason to live, your only love. You share his goals, his ambition to turn the whole world into an evil place he will rule without question. Everybody shall obey him, and anyone who tries to oppose the Dark Master shall be destroyed - or, better, turned into another loyal slave. You were a fool to think your little trick could work on him ... or on her ? You look at the Dark Master, and you can't tell what their gender is anymore.
Because it seems the merging process was at least partially successful. The Dark Master is not an hunky man anymore: they are still muscular, but they look like a amazonian woman, with E-cup breasts and an hourglass figure. Their face is female, with long hair going down to their rear. Only their genital remains untouched. The Dark Master is now a powerful and beautiful shemale.
You kneel before them and suggest them to take your life for the offense of turning them into a shemale, but they reply: "Interest development. Trying to merge with me like that was brilliant. I must say you almost got me. I can feel the need of being my own female slave. That's a weird thought. And you also drained a small part of my power."
So you weakend them, even if it isn't significant. The old you would consider this as an achievment, but the new you view it as a blasphemy. The Dark Master is your God, so you strongly believe he shall kill you for your blasphemy. And if your master takes your life, you won't resist. At this instant, the third seal is broken and you regain all your magical powers, and even more. You have absorbed only a tiny fraction of the Dark Master's power, but that is enough to double your magical abilities. That's how you realised how powerful the Dark Master really is. No one can challenge them. And yet, you were foolish enough to try.
But they have other plans. They say: "I won't take the life of my smartest slave, you are more uesuful alive than dead. In fact, what you have done may be an improvement. What do you think of the new me, Ebony ? Are you still attracted to my body ?"
Ebony fondles the Master's magnificient boobs and gently kiss their lips: "You are more sexy than ever, Master. And if you decide to become fully female, I'll be eager to eat your pussy."
The Dark Master nodes: "You read my mind, Ebony, but we must first reward your friend here. She deserves a new name. She shall be known as Amber. Amber the witch."
You acknowledge your new name: "Thank you, Master. Xenor Tarkal the Shadow Mage is no more. I am Amber the Witch, and I live to serve you."
You offer your body to your shemale Master. Your breasts are squized by the Master's own boobs as you impale your pussy on the Master's cock. Sex is even better than the first time. Maybe because you are now completely devoted to the Master and let them use your body as they please, or maybe because the Master is now partially female. Or both. Anyway, you get 10 orgasms before the master turn themself female and Ebony, the Master and you swith to a lesbian threesome for the night.
At dawn, the Master reverts to being an amazonian shemale, which seems to be their new default form. You politely ask them if you shall call them "Master" or "Mistress".
They answer: "Call me 'Master', at least for a while. I am no longer bound to a single gender, and after have complete your mission, maybe you will call me 'Mistress'."
"My mission ?" you ask
The Dark Master precises: "Yes. I have a special mission for you, Amber. You shall ..."