An amanita

You had decided to combine her with a fly amanita. 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. The ritual could begin.

You collected a fly amanita mushroom and placed it on the woman's stomach. 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 stomach became like liquid, allowing the mushroom to disappear into it. While it was absorbed by the woman's body, she started to change.

Her skin changed color and became pale. Her hair took a reddish color and a large red mushroom grew on the top of her head. It was like she was wearing the mushroom as a hat, unable to congeal the long hair that draped along the sides of her face.

Finally, white spots covered the top of the giant mushroom on her head.

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

You had turned the woman into a mushroom-girl.