Light.
That single word burned itself into the sky, quite literally, as giant luminescent letters hung in the air and produced the very thing that those letters spelled out. Light. No longer was the battlefield obscured by an oppressive darkness, only being held at bay by a few dimly flickering torches. Now the world was completely bathed in light, so that even the furthest reaches of the hordes back line was completely visible from the defensive line of the library.
There was also the benefit that this luminescent word, which projected itself like a divine decree from an all powerful god, had blinded anyone unlucky enough to be looking in its general direction upon its appearance. Not permanently blind, only for a few seconds at the most. Like a flashbang without the bang, so just a flash really.
