Complex sentence: The kakugane is an alchemical device that, when activated, takes a form based on its user's personality, forming a buso renkin--the only thing that can destroy a homunculus.
Split into simpler sentences: The kakugane is an alchemical device that, when activated, takes form based on its user's personality, forming a buso renkin. The buso renkin is the only thing that can destroy a homunculus.