Even Ombric was rendered momentarily speechless. But the wizard now understood what they were all about to face.

“I suggest, given current events, that the safest place for the children to sleep this night is Big Root,” he said finally. “An ancient evil has awakened—and I must tell you more. Come.”

And before anyone could agree or not, the wizard threw open his cloak and transported them all to the tree

Nicholas Saint North and the Battle of the Nightmare King Chapter 4