Knowing how your girlfriend liked accessories, you chose a jewelry necklace for her. It was not very ornate but still very well made, being all gold and containing four precious stones set in it, a blue sapphire, a very red ruby, a very polished green emerald and a diamond, all precisely cut and set in the shape of a diamond. You took it to the cashier, paid for the item, and asked them to gift-wrap it. The cashier lady said that it was a curious choice, but that the person who received the gift would like it.
With the purchase completed, you went to your girlfriend's house. As soon as you arrived you went in and handed the gift to her, she unwrapped it and marveled at the jeweled necklace.
"Oh you are a sweetheart. I loved this necklace." your girlfriend, Alice, said
"You deserve it." you said
"Even so, it must have cost a fortune. Just look at these gems." Alice said with her finger touching the blue sapphire, which you thought you saw a slight glow, but thought it was just the light reflecting off the stone.
What happened next was quick, your girlfriend's tights were destroyed along with her panties as her legs quickly fused together and became a fish tail.
"What? Did I become a mermaid? Is that the thing about the necklace you gave me?" Alice said
"I'm not sure, but I had the impression that I saw a slight glow when you touched the sapphire and then you transformed." you said
"You say when did I do this?" Alice asks and touches the sapphire again, which again shows a light glow and soon after your girlfriend's fish tail turns back into a pair of legs. "Am I back to normal? This necklace is magical. I felt some inclination towards the water when I touched the sapphire, maybe this turned me into a mermaid, when I touched it again thinking about my normal self I went back to what I was before."
"So that's what the saleswoman meant by interesting. Maybe you have more, try tapping on a different gem now." you said
Alice then touched the emerald, which also exhibited a slight glow to the touch. Soon her legs fused together again and became covered in scales, then elongated into a long snake tail. Alice is a Lamia now. "Cool, I felt a connection to the land. Let me try something." She said and took off her skirt, then touched the emerald again. Soon four horse legs grew out of her serpentine tail, which thickened and covered with black fur, and the end of the tail reduced to a horse's tail.
You watched with a mixture of excitement and surprise as your girlfriend transitioned from a lamia to a centaur. And after moving for a minute like this she touched the emerald again, she transformed again back to normal.
"I think I'm getting the point, sapphire allows me to transform into whatever I want as long as it's related to water, and emerald does the same for land." Alice said
"Incredible. The other stones must also have a similar power. Try the diamond now." you said
"OK." our girlfriend said and touched the diamond. After she did this her legs became covered in feathers as well as her arms which became wings, her feet became eagle feet as well in her transformation into a harpy. "This allows me to become creatures that are related to the air, more specifically those that can fly." She said, touching the diamond again and transforming again, this time with the feathers falling off and being replaced by red scales, her wings moving to her back (destroying what was left of the clothes she still wore) as a new pair of arms grew in place. A few horns grew on her head as well as spikes on the end of her new reptilian tail. She had become a dragon girl. Once more she admired herself before touching the diamond once more and returning to normal.
"Now all that's left is to try ruby. What will come of it?" you said
"We'll find out soon." Alice said and touched the ruby. As she did so her eyes turned completely red like stone and a huge smile broke out on her face. A pair of black wings grew on her back as well as a tail with an arrowhead, a pair of horns also grew on her, her breasts got bigger and she gained more curves on her body. Without a doubt, your girlfriend had become a succubus.
"Wow, you are incredibly sexy." you said
"And I have more than that. That information that popped into my mind, I don't just have the looks of a succubus, I have their powers as well, so do their desire for sex."Alice said and, with a quick movement of her wings, she flew to you, kissed you and took her hand between your legs, stroking your cock and making you hard. You then saw a glint in her eyes and then heard a noise behind you, which you turned to look at and saw that it was your empty clothes that had fallen to the floor and that you were now naked. You understood that as a succubus your girlfriend had magical powers of her own and had teleported your clothes off your body.
She wanted that and you gave in, letting her guide you to lay you down on her bed, and she sat on you making your cock penetrate her pussy. It was never your intention when you gave her the necklace, but you didn't regret it either, you just fucked her until you came. Whether or not it was because of the necklace's power, you both agreed it was the best sex you'd ever had.