Which was all very well, but what on earth should she do? What would Mr Sherlock Holmes do? He would say, ‘This is a very singular three-pipe problem, Watson,’ and then he would…

She stopped.

Problem.

That had been the word the Midnight Swan had used.

Suddenly an idea went right through her so fast it felt like a bright light. She drew herself up. She turned to face the sinister masks of the Fair Family.

‘I made you a promise.’

‘Yes.’

‘I promised you I’d come if you want me to, were the exact words I said. But maybe I can offer you something else instead, something you’d like better than me.’

The Tylwyth Teg giggled and hissed. ‘What else would interest us? What else would we want…’

‘My schoolmaster taught me about you.’ Seren glanced at the Crow as she said it. ‘There are two things you always hunger for. A human child is one. The other is … treasure.’

At the same time she unbuckled the collar of diamonds from under her coat and laid it down on the shiny wooden floor of the ballroom.

The Tylwyth Teg gave a long, hissing gasp.

How the diamonds shone! They were like small facets cut from the face of the moon. They were the brightest thing in the whole brilliant room. They took light from everything and glinted it into rainbow shimmers that played over the narrow eyes and greedy hands of the clustering faery creatures. They were the tears of the Midnight Swan.

Seren said, ‘You can have these instead of me. But you must go away and leave us all in peace from now on. No more trying to steal us away. No more trying to get into the house. Peace between us. For ever.’

‘Peace.’

It was a whisper of awe.

‘Yes. If you agree with all that you can have the diamonds.’

‘You can’t trust those creatures,’ the Crow hissed in disgust. ‘And what a waste of such a gift!’

She shrugged.

The Tylwyth Teg looked at each other. If they spoke she didn’t hear a word. But they must have a secret way of talking because they all nodded their heads one after another, all the beautiful, ugly, strange and sinister faces. Then one of them, a small dancer in a mouse-mask, bent and picked up the diamonds with a shiver of delight.

The shiver spread. It rippled through the crowd, through the mirrors, through the chandeliers. It rippled through the curtains and the floor. It rippled through Seren like the cold feeling you get when something delicious scares you.