"I'm a mermaid." she said. "Larissa: Princess of Lemuria; as are you."
"A mermaid?" Ty repeated with a look of obvious skepticism on his face. "Aren't they supposed to have fish tails, and live in the sea?"
"Have you not heard stories of mermaids that can take human form on land?"
She had a point, a great deal of mermaid stories he'd seen had such a thing; and he'd seen plenty, being a big fan of them; but Ty still wasn't entirely convinced. "Alright, let's say, for a moment, you're telling me the truth. Why did you give me up, and why are you here now?"
"When you were born, there was a prophesy, of a mermaid who will come into a power greater than any other. One who will challenge a coming darkness that threatens the oceans of our world. That prophesy was about you, Ty."
"Our World?" Ty asked.
"Magical beings like us originate in an alternate world we call 'Gaia'. Mermaids rule many of the major oceans of that world. Our ocean is called Lemuria."
"The prophesy also warned of threats against your life in the meantime, so we decided the best thing to do was turn you into a human." A tear ran down her face and her voice started to break up. "It was the most difficult decision of my life." She wiped her eyes as she took a moment to recompose herself. "We placed a powerful seal on your mermaid form, one that would not be broken easily, but you would be able to undo when you came into your power."
"How I have waited for the time that we can return to the sea together!" She gave Ty a big hug. "Oh, how I've missed you so much."
"So, not only am I a 'chosen one', but a prince of a parallel world's ocean to boot?"
"Prince? Oh... I assumed you'd figure by my use of the word 'merMAID' that... um..."
"Wait, I'm a GIRL!"
"All our kind are, making you a boy was an extra precaution in case our enemies ever got wise to our plan."
This was a lot to absorb for Ty, as Larissa escorted him along. Lost in his thoughts, he was unaware of where he was headed, until he felt sand in his shoes. Ty looked up to see he was at the shore.
"We're here." Larissa said before snapping her fingers and suddenly appearing in a bikini made of seashells and a silvery bikini bottom. He slowly approached the shore when Larissa stepped out into the water and laid down. As the waves washed over his mother's legs, Ty saw the bikini bottom take on a more scaly texture, squeeze into her hips and fuse with the skin. The scaly skin continued travelling down her legs, pressing them together, the definition of knees and leg bones waned into a smoother, gradual taper. As the effect got to the feet, Larissa raised them into the air and shook them vigorously as they thinned and flared out into a large fin.
"Now, it's your turn." Larissa said, "Undo your seal, become my daughter once more and fulfill your destiny."