A few weeks prior to Basil's trip back to Woster County — roughly after his return from Lone Mountain — Deacon had told something to him. He had said he had been feeling something weird happened to his body.
Being an experienced man, Basil had instantly figured out what had happened to him. It was the process of Evolution. Finally, Hormonal Rush — the nurturing pill Basil had given Deacon — had taken effect.
The hard fight with Yamata no Orochi might have also played a big role in pushing Deacon's Evolution. Hormones might be able to push the growth of a Magical Beast forward, however external stimulation was also needed.
After facing a life and death situation, Deacon's instinct to survive might have gotten activated, thus pushing his body forward to suit his harsh living condition. It would then lead to Evolution.
