"What is it?" you ask. You worry pain may be involved, or complicated magic requiring rare ingredients you wont be able to find or afford.
"To transform into a merman or mermaid, a human must have sexual intercorse with one. Sex with me will transform you into a merman. Once transformed, if you have sex with Caroline she will become a mermaid."
Your eyes light up. You'll get to have sex with Topaz and Caroline? This is a bargain you can live with.
"I have no problem with that" you say, clearly excited.
"I hope your friend does not either. I am aware human females can sometimes be very jealous about such things."
The doorbell rings. "That must be Caroline. I'll got let her in and we'll get you out of here."
You go get the door...