Power of Three. Diana Wynne Jones

He pointed to the nearest, with its rounded stone hood and the twig-shaped writing on the stonework, which was the indoor equivalent of a thorn tree.

      “I hope you’re right,” Orban said glumly.

      Gair looked from his uncle’s weary face to the tears running down his mother’s. In a shocked, distant way, he knew there had been a terrible disaster. War, he thought. But it did not feel like that. He could not imagine Otmound as an underground lake or think of more than one Dorig at a time. As for Og, it was a shame, but to Gair he was a fussy old grandfather whom Gair had not known very well, or to tell the truth, liked very much. He looked at Ayna and Ceri’s sober faces and saw they felt the same.

      The important thing to all three was that here was Ondo back again after only two days, and the important question was when was he going?