You nod weakly, a little afraid of refusing, not only worried you'll be passing up your only chance to escape, but also afraid of what Alexander might do to you if you don't say yes. "Al-Alright, I'll let you be my companion. Are there... any magic words I need to say, or...?"
He seems to let out a sigh. "No no, nothing so convoluted. I just need you to open up your mind and your magic to me so that we can become connected. It's perfectly painless, and I assure you, you'll probably even feel better after it." You're not so certain, but you do your best to close your eyes and relax, trying not to resist any sort of outside force. Soon enough, you feel your body become more at ease in his grip. "Yes... Just like that..." When you open your eyes, you realize more than just that is happening, though. He's gotten smaller... younger, you realize. As for you... you watch in shock as your tail starts splitting, again and again until there are eight long tentacles extending from your lower body.
"What... What happened? Now I'm an octopus? You didn't say that was going to happen!" You try to fold your arms, but somehow get confused with all the other appendages you have now, causing a couple of your tentacles to flail a bit.
He pushed himself up off the floor, much more mobile now that he was considerably younger. "I didn't think it was relevant under the circumstances, but mermaid witches become a reflection of the species they take as their familiar." He pushes you aside, finally letting go of you. "Mmm... Feels so good to have some real power again. And I can feel that Tanner spark. Should make this really simple." He glances at you. "So much potential. Glad you didn't have the chance to become a real witch, or else you might have actually been able to cause some trouble."
You're about to ask what he means when he suddenly darts out of the hut. You feel sick to your stomach unlike anything you've felt before. Some sort of sign of how bad a mistake you made? You try to follow him, but it takes you several minutes to figure out how to use your tentacles to propel yourself. All the while, you keep getting odd feelings in your stomach. When you do finally make it out, you look all around to see if you can find the octopus, but he's nowhere in your range. At least... until you notice a face peering in at you from the outside, a male face. You now realize why you felt sick. That was him using your magic to change back into a human.
"Well, Coral, I have to thank you for so generously allowing yourself to become /my/ familiar. I never would've been able to free myself without your help. I have a little problem, though. You see, I have access to all your magic as long as you're alive, but... if Marisa finds out what happened, she might be able to break our connection. We don't want that, now do we? That's why I'm going to..."