BxB Gasper's power

Gasper’s body was shrouded in shadow and turned black. And then, Gasper, whose body turned black, changed his form. He changed as his body got bigger and expanded, surpassing the height of the Devil who attacked him by far. What appeared there was a giant black monster. Being five metres high, it resembled a Dragon closely as it emitted a sinister aura. The whole team of unknown Devils seemed to tremble in fear due to the amount of the aura emitted.

Gasper was without a doubt one of the strongest members here. After going through a lot of things, the Sacred Gear [Forbidden Balor View] that Gasper owned awakened and changed into a completely unknown Sacred Gear. —[Aeon Balor], that was the name of the Sacred Gear that Gasper currently owned.

True DxD Volume 1, Life 2