“Ha. Yes, well, in a way it is. Walpurgisnacht is the night between the last day of April and the first of May. Tomorrow night,” his grandfather said, searching for a solid spot to anchor his grappling hook.

“It was named after an English saint. St. Walpurga. She came to do holy work in Germany during the eighth century. Interesting stuff. Some legends say that if you speak her name to an angry dog correctly, it will calm the animal. Anyway. Walpurgis Night coincides with a point in the calendar that’s been considered important for thousands of years in many countries—people giving thanks they’ve survived another winter and banishing the last of it to welcome in spring and summer, you know.

“But around these parts, it’s all rather more . . . potent. Some also call it Hexennacht: Witches’ Night. The stories go that Walpurgisnacht was when witches would gather on the mountains for, well, strange celebrations, like the fellow said. A second Halloween, if you like, when ghosts and ghouls run free.

“Just as there are places where things get thin, Alex, there are times when things get thinner, too, when half-remembered old beliefs come bubbling up to the surface. Tomorrow night, in this area, you’ll have both things happening at once: a thin place at a thin time.

“And all of this worries me.” The old man paused to secure his rope around a tree trunk. “The timing of it, the way it all seems centered on the Black Forest. This place, at this time. There are coincidences in the air I don’t like. The story about the mountain disintegrating, the old Shadow Gate legend about a castle falling. Two great unexplained collapses.”

“Could it have been here? The Shadow Gate? Is it connected to the collapse of the Devil’s Pulpit?”

“Hmm? Oh, no. I mean, the Shadow Gate legend dates from, what, five, six hundred years earlier, Alex. And it was supposed to have happened in a castle somewhere.”

“Could that be it, then?” Alex said, trying to fit pieces together. “If there’s a code in the paintings, maybe it tells you where it happened. Where the gate was. Like, I dunno, you . . . follow a path through the locations in the pictures, or where they were painted?”

“Decent thinking, Alex. I had thought something similar.” The old man grunted, still fussing with the rope. “But it’s led me nowhere yet.” He straightened with a puff and slapped his hands.

“Now,” he said. “There’s just a little bit of climbing to do. Need to get down there.” He pointed opposite their bench, where the ground simply dropped away. Alex stepped gingerly over and peered down: an almost vertical cliff of jagged granite.