This might actually be interesting, Liska thought, sitting up in her chair. Across from her, the pasty, brown-robed boy flipped open a notebook and waited with his pencil poised.

‘Hmmm. Well . . .’ Master Ambrose ran his fingers through his beard several times as he thought. Liska noticed he wore rings on all his fingers, each with a softly gleaming lump of amber set inside. ‘The answer,’ he finally said, ‘would have to be . . . “perhaps”. There have been many pairs of such trees before, you see. It seems to take three to four hundred years for them to fully grow. Then the process begins in which the shadow tree sends out its minions to attack the tree of light, attempting to destroy it.’

‘But can’t the shadow tree be stopped?’ the mossherder asked again. ‘Can’t we use our magic?’

‘We are indeed trying to use our magic,’ said Ambrose. ‘There are whole sections of the temple whose only purpose is to find a way to destroy Bitterblight before it becomes too powerful. And if it does reach its peak – should that day come – there is always the last resort.’

‘What’s that, then?’ The mossherder’s hand was still up and she was now getting lots of angry stares from the children in front. She didn’t seem to notice.

‘Well, the last resort, as I thought you would have known, is for someone to take the heart of the Undrentree – the glowing seed at the very centre of its trunk – and plunge it into Bitterblight’s core. The shadow tree will then be destroyed completely, as will all its hideous creations.’

‘Then what will happen to our tree?’ asked the mossherder, her voice quivering.

‘Ah,’ said Ambrose. ‘Then, unfortunately, our tree will die. And the city will die with it. Of course, this is very unlikely to happen. We are bound to come up with a way to stop Bitterblight before then.’

Liska’s mind flashed with the scene from the other day – the argument on Undren Square that had landed her in so much trouble – and before she knew it, her own hand had gone up.

‘Is it true that some of you mage-folk are planning to use the powers of Bitterblight as a weapon?’ There was a sudden, complete silence in the classroom. Her voice seemed to echo about the rafters and every single pair of eyes turned to stare at her again, to burn right into her flesh. The arbomancers, in particular, looked livid with fury. A bulky, scowling boy at the front was going a ripe shade of purple.

‘A weapon?’ Ambrose squinted in her direction, his head turning to and fro like a blind mole, popping out from its tunnel into the sunlight. ‘Wherever did you hear that?’