A butterfly

You had decided to combine her with a butterfly. You had the woman strapped naked onto a table. She was a little difficult at first, but after you cast a dizzy spell on her she was much easier to handle. You had expected many transformations to happen on her backside, so you had her lie face down. The ritual could begin.

You collected a butterfly and placed it on the woman's back. You placed your hand over it and summoned your magic. This put an ethereal glow over your subject. The woman started to become worried and asked "What are you doing?"

You chose to ignore her and continue the spell.

"Stop it." She said softly.

You continued to ignore her and persisted the spell.

The woman began breathing heavily. She felt something was happening to her.

"Stop it! Stop it, please! Somebody, help me!" She tried to say. She was too dizzy to resist at all and was completely at your mercy.

Finally the preparations were complete. The merging could begin.

The woman's back became like liquid, allowing the mushroom to disappear into it. While it was absorbed by the woman's body, she started to change.

The first change that happened were two antenna sprouting from her head. After that two large bulges appeared on her back. Eventually, the skin on these bulges broke open and two large wings grew from her back. They were as large as she was tall, with each of them as wide as her length. You were surprise at the wonderful colors they contained. They were so beautiful.

Once that was done, the merging was complete. You ended the spell and retracted your hand.

You had turned the woman into a butterfly-girl.