Futa on The Elixir

He's hailed as the village's mage prodigy and can barely cast a spell, so he sneaks into the forbidden woods every day to trade chores for real lessons from a secluded witch. The tone is short and blunt rather than sprawling.

The trouble starts when the witch drinks her own experimental elixir, supposedly to teach him faster. What comes out of that is not a demonstration spell but a full-body transformation that flips who's actually in control of these lessons, and the boy's carefully kept secret stops being the biggest problem in the room.

There's no branching path here: it's a kinetic novel, so you read through the fallout rather than steer it. The story moves in one direction, from a nervous apprentice hiding his failures to a student getting a very different kind of education, with the witch firmly setting the pace throughout.