You make your choice, and gulp down the first half of the potion. You pass it to the woman, who gives you the same strange smile and drinks the second half. The potion seems to flood through your body, and you can feel it flooding through hers, too. She walks toward you, submissive and slow.
"You are the master, now," she says in a calm voice. "You only have to imagine us merging, imagine us joining into one body, and it will become so. I can feel my body's form weakening. I can feel my desire to be independent loosening. This potion does things to your body but also to your mind. I just want to merge with you, in whatever form."
You hadn't noticed it before, but when she says that the potion affects your mind, you become aware that you feel dominant. You feel powerful, a being that can control the form of another being. She is subservient to you, and in your mind, you deserve to be this powerful. It is an unnerving sensation to see another human being as beneath you. It occurs to you that soon enough, you will feel submissive to her, and a quick pang of panic rises in your mind before it is pushed out by how powerful you have become.
You look at her, at her beautiful body and her thrilling curves. Yours, all yours. There is no she, there is only what the two of you will become.
A few ideas are in your head. You could merge so that her mass joins you as muscle, so you are hulking and mighty. You could merge so you are attached but still able to copulate - the thought of feeling the feminine orgasm excites you. You could make her a mere body part - a large penis, perhaps, or a pair of big boobs to play with. There are so many options.