Complex sentence: Despite Walker's claim that the power of the blue light is the greatest in the spectrum, Atrocitus reveals the weakness behind the Blue Lanterns: hope is nothing without willpower to enact it.
Split into simpler sentences: Despite Walker's claim that the power of the blue light is greatest in the spectrum, it's then that the reader is made clear of what Atrocitus claimed to know as the weakness behind the Blue Lanterns. Atrocitus points out that hope is nothing without willpower to enact it.