Instead of a human being looking at you from the mirror, there is a small, black bird. ...Wait a sec. Doesn't this mean that...!
You look down at your body to see that it's no longer a human form. Instead, pitch black feathers cover your skin, and your feet are now sharp talons that could attach you onto any object flawlessly. When you attempt to stretch your arms, you see that your arms are wings, covered in those same black feathers.
...The words "What the hell" are self-explanatory here.
When you attempt to shout out for some help, you're cut off by your own screech, which is now that of a crow's. It's ear-piercing, and you have to now just keep quiet. Keeping your thoughts to yourself. ...Besides that, from the mirror, you can see that your mouth has shifted into a beak, and that your eyes are now a dark gray.
You look left and right, and then you hear a door opening. It's creaking, unpleasant to the ear. A lot more pleasant than your own cry, though. From the bars of your cage, you can see that same young woman from yesterday approaching. She crouches down to the curious you.
"It seems my spell was a success!" The young woman jumps in victory, as if she didn't expect this to happen? "Oh, right."
It was then you heard that same voice inside your head, echoing. "Time for exposition- I am Iris, an Apprentice Magician. I was recently told to get one of those 'Familiar' things, and I used a summoning spell to bring you right here. ...Not to this prison, to that forest you were in. It was then I used a spell for marking a Familiar, and you just fainted on the spot when the transformation occurred."
You were not surprised to hear that you did. It's kinda strange that you didn't faint. After all, don't victims of transformation curses or whatnot often wind up staying awake through the whole thing? You then angrily said (thought?) to the voice of the Apprentice, known as Iris, that you did not expect this at all. And you are not happy about it.
Iris shrugged. "Eh, you'll get used to it eventually. It's normal for familiars to freak out at this sort of thing at first. So!" She instantly changed the subject over to your life from now on. "As a Familiar, you have to serve me as my pet! ...Or Servant, but I prefer the term 'pet'!"
You don't blame her, seeing the implications of the word "Servant" nowadays.
"...Oh, and you shouldn't forget this important detail. It's kinda vital to both of us. Y'see, if I die, you die, and if you die, I die. That Familiar spell linked our souls together, and that's what causes it."
Well, damn. Iris then unlocks the small door at the top of the cage, extending her index finger. "Since you're not used to this body yet, I'm going to give you a hand. Just... Make sure not to fall."
You reluctantly agree to this, and hop onto Iris's index finger. It seems she's leading you somewhere...