The spell was supposed to be simple, but the focus requires a true name. Without it, 'mana' just erupts. Luciel wouldn't have done it in any other situation, but he was convinced that righting his mistake was more important than his own short-term health and comfort.
So, the burst could arguably trigger William whose latent powers could seep out from the flood of energy permeating the room.
As for Gish, the language and essence-tinge of the spell would certainly be all too familiar.