One of the most intriguing alchemical legends is that of a magical stone that is believed to capture and transfer the very essences of things. The only evidence of its existence are ancient writings telling about it, and historians still argue over whether or not these writings refer to a real thing, or merely some metaphor.
They're wrong. The stone exists.
The transfer has the wondrous power of transferring a quality from one thing to another. In this it maintains the alchemical principles of conservation - nothing is created or destroyed, merely moved from one place to another. Alchemists originally wanted to use it to transfer the essence of gold into base metal. What they got instead was something far more wondrous.
Exactly what the stone transfers is up to the wielder's imagination, limited only by their ability to think or say that they want to move a certain essence. It can transfer concrete qualities like weight, height, age, the size of body parts, and so on. Or it can transfer more abstract essences, like humanity, sexuality, knowledge, and gender identity. What does it look like to transfer 40% of a man's masculinity and move it into a woman? What is it like to lose 63% of your humanity? Nobody knows... yet.
The stone is easy to use. The wielder has but to hold it and have a target and a recipient visible near them. Then they have to announce - mentally or aloud - that they want to transfer a certain amount of something from the target into the recipient, and the stone works its magic. Contact is unnecessary, but it can be done if desired.
If there is no recipient to take in the transferred essence, the stone instead absorbs it into itself. But it cannot do so for long. After exactly one hour, any essence the stone has absorbed will be released into the nearest recipient - often the wielder. And trying to absorb more than one essence will result in the same outcome.
The stone has its limitations. It cannot do anything that violates the laws of physics or biology. Most importantly, it cannot kill. It cannot erase someone's personality or mind. It can grow or shrink people to the size of giants or ants, but it cannot make someone grow so big they are unable to breathe, or shrink so small they wink out of existence. If it reduces someone's age to zero, that person is immediately sent into the nearest viable uterus, to grow and be reborn as a normal baby - one that will remember their previous life, for better or worse.
In appearance and size the stone is roughly like an egg, dull tarnished gold in color with a slightly rough surface. It feels heavier than it looks. Its similarity to gold means that many have greedily obtained it, only to discard it afterwards.
The stone has changed hands many times over the centuries, crossing land and sea to end up in the most unlikely place possible. And it is found by the most unlikely person...