Grand Gale was the sole Primary attack function of Aizel's Territory. The woman had decided that she didn't require a Secondary attack function. Thus, using Creeds, she instead sacrificed her Territory's ability to generate Secondary functions entirely, in order to create a Primary function that was strong enough to end her battles as soon as she invoked her Majestic Territory.
Because Primary Territory functions couldn't be anything too complex, Grand Gale was as simple as it looked – scorching lightning that fell continuously on a target for ten full seconds until they were ash. But a feat like this could only be accomplished by experts with Territories that had living beings to help amp up the power of attacks.
And thus, predictably, Yuyui met her death.
'I suppose I should have expected it. They aren't nobodies,' she thought as branches of the lightning in the aftermath traced along the ground she stood on.