As the darkness hits, it hits Strange like a truck. Almost instantly, the magic stops. The vines wrapped around the Mother stay put—but at the moment, Strange isn't making them grow any more.

He's been in this before. Trapped in a pillar of darkness, something that slowly tried to kill him, and would have killed him had he and Norrell not killed the enchanter. That's got to happen here: they've got to kill this fae. He didn't give up then, Strange refuses to let himself give up now, despite the thousands of voices in the back of his head that are trying to tell him otherwise.

So Strange tries to push past the fear and the worry as he shoots a jet of fire high in the air in what he thinks the direction the Mother of Invention is in, in a valiant attempt to push past the darkness.