This was the power surge from the deaths of numerous demons converging upon him. Initially intended as a 'backup' for the demon generals to maintain some level of power to perish alongside the enemy even after losing their army, the power from the sudden demise of seventy percent of the demon army overwhelmed Sorrow Bell, shattering his perfect composure.
[You monster!]
Driven by rage, extreme shock, and the bitter hatred from his failed mission, Sorrow Bell's demon transformation intensified suddenly. Screeching, he charged at Ian, with the infernal flames burning in his three deformed and mutated eyes.
But Ian would not answer those questions, instead, he laughed heartily and once more invoked the power of the Heart of Terra to empower himself.
