Trial of the Grasses
Set a unit's power to 12. If it's not a Witcher, damage it by 6 first.
Imagine a lump of clay. In order to shape it, you must first moisten it or it will crumble. The Trial's initial part does just that. It opens the body to change, so to speak. Only then can the mutagens produce a witcher.
Illustration by: Bartłomiej Gaweł