A second's hesitation is enough, even without Lambert's panicked spell casting, though he may want to step back to avoid what's coming. Blue and black flame, striking Childermass in the back and going straight through, not entirely unlike a lance, though that it will leave the faint smell of singed cloth and feathers is unavoidable. No singed flesh, however, which... is probably good?

Regardless, it has him giving a pained jerk in the witcher's grasp, a gasp, and both Lambert's magic and the Count's command falling apart against his mind because, well, basically everything just goes dark. Unconsciousness is a blessing right now, just try not to let him hit the floor.