A black cat

You had decided to combine her with a black cat. 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 some black cat-hairs and placed them 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 cat-hair to disappear into it. While the hairs were absorbed by the woman's body, she started to change.

First was her hair color, which changed to black.

Then came her ears. They moved up her head and became pointy. There also grew hair on them.

She grew a long thin tail. Black hairs also grew on this one.

Her fingers kept their shape, but her nails turned into retractable claws.

The last thing to change were her eyes. Her irises became yellow and the pupils elongated.

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

You had turned the woman into a cat-girl.