Boost a Witcher to 25 power; or Deal 10 damage to a non-Witcher unit. If it survives, boost it to 25 power.
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.