Once Bundal's arm underwent such a drastic change, Kieran's view of the situation changed subtly. Though it wasn't confirmed, what Bundal just did reminded Kieran of why Isadora's ability sounded so familiar after Agatha mentioned it.
'Transmutation! That's why it sounded so familiar. A normal Alchemist is ineffective on a battlefield, but one who learns the Lost Art of Transmutation can become a force to be reckoned with, even on the battlefield. However, learning even one principle of it is more difficult than Awakening…'
Transmutation, in a nutshell, was the ability to go against the natural order by obscuring the laws that governed reality.
However, these changes were finite in duration and effect. But of course, the duration, effect, and scope of the transmutations were not constants.
They could be manipulated.
Regrettably, manipulating these aspects of transmutation was a very demanding feat.
