There was a sharp intake of breath from the brownies.

‘We told you not to trust her!’ said Dubbeljøk.

‘You can’t be serious!’ said Archie, staring at the woman in disbelief.

‘Why not?’ Mrs Puddingham-Pye clutched the jewel tightly. ‘I should be the Guardian of the Wyrdie Tree, not you. I should have access to all its power. I could use the jewel to strip the Tree if its magic and bind it to me. Think of what I could do!’

‘We are,’ muttered Billy. ‘But whatever you do decide to do, do it quick – the Mirk isn’t going to wait around for you.’

The monster had almost quenched the blaze, despite the combined efforts of Archie, Billy and Fliss’s Fizzfires, and the supportive barks of Sherbet. It clambered through the smoke and fog, over the ashes of its army, and angrily lumbered towards the tree.

‘I’m the Guardian, and I can’t change that,’ said Archie to Mrs Puddingham-Pye, thinking fast. ‘You could take the jewel for yourself, take all the magic, but then what? Eventually the magic will run out. You don’t have access to its source, deep in the earth, like the Tree does. The Mirk will come for you and what will save you then? I won’t be able to help. We must work together. There’s no way either of us can destroy the Mirk alone – we need each other, you said that yourself, or else there’ll be no magic for anyone. Please – give me the Treeheart. I’m the Guardian and it’s my job to do this.’

Mrs Puddingham-Pye was silent for a few agonising milliseconds. Then she tossed the jewel in Archie’s direction.

‘I wasn’t really serious,’ she said, with a casual sigh, though Archie had caught her shiver at the idea of the Mirk coming after her, ‘but it was amusing to wonder at what might have been.’

Archie ran – the Treeheart in one hand, Fizzfires in the other – up the slope to the Wyrdie Tree. Its branches were now completely bare, with the exception of one lonely red leaf. Any second now, that leaf would fall and the Tree would be a skeleton of itself, vulnerable, unable to renew until the Treeheart was in place. The Mirk knew this as well.

It was the monster’s last chance. With a horrible growl, the Mirk charged at Archie, trying to cut him off before he could reach the Tree. Archie was almost there, but the monster was easily gaining on him. It leaped at the boy, its black, bony claws stretching out to seize him. A bolt of blue lightning shot from Mrs Puddingham-Pye’s broom and knocked the Mirk out of the air, sending it crashing into the earth with a terrible howl, just as the last leaf gently floated to the ground.

For a moment, there was silence. The Tree stood starkly against the sky. It did look sad and defenceless for something so huge. Archie knew he had to protect it. He took a deep breath and slammed the jewel into the notch in the bark.

‘I’ve done it,’ he gasped with relief.

The Tree shuddered, as a green light spread from the jewel through the cracks in its bark, like blood through veins. As it flowed up the Tree and streamed around its many branches, buds appeared, which instantly burst into leaf. On and on the light went, further up to the topmost boughs. Soon, the Wyrdie Tree was covered once more in a vibrant, vital green that shone in the evening sun.

Archie could feel the Tree’s magic growing with it. All his fears and doubts disappeared – it was as if he were recharging along with the Tree. The Mirk hissed and spat angrily, looking at Archie with a boiling hatred.

‘You may have beaten me,’ it growled, ‘but I’m not finished! I will return in another five hundred years, then again and again until the end of all things!’

It planted a claw on the ground, sending black toadstools and fungus crawling towards Archie, killing all the grass as it surged forward. Mrs Puddingham-Pye fired her broom at the Mirk, but whilst it held the monster back, it wasn’t enough to stop it.

A thought appeared from nowhere in Archie’s mind. Fight darkness and decay with light and life. Was the Tree talking to him? Or another voice – was it Belle McBudge, giving him one last piece of help? He raised his hand, pointing at the Mirk. A tree root erupted from the earth in front of him, driving through the fungus and wrapping itself around the Mirk like a snake, squeezing and holding it tight.

‘He’s properly wyrdworking!’ said Billy delightedly.

‘Archie!’ said Fliss. ‘Are you making that happen?’

‘Me and the Tree and Belle McBudge!’ said Archie, frowning in concentration.

‘You heard what the boy said earlier,’ called Jøkchip to Mrs Puddingham-Pye, who stood watching Archie with surprise. ‘It’ll take both of you to destroy it.’ She nodded and sent lightning from her broom into the Mirk’s body. Then Blossom joined in, breathing flames at the monster, as Billy and Fliss hurled more Fizzfires at it. The Mirk writhed and howled under the united attack, and began to shrink before their eyes.

‘It’s working!’ shouted Billy. ‘Go on, Archie!’

The root coiled tighter and tighter, trying to crush the darkness. The Mirk fought back but shrank more and more, its body gradually weakening and fading.

‘I may fade,’ it shrieked at them, ‘but you cannot destroy me! Not whilst the last piece of Mirkthorn remains.’

‘Oh, yes we can,’ said Fliss. Calmly reaching into her tatty, fire-scorched bag she pulled out a toy – a black, thorn-covered puppet. She dangled it in front of the Mirk, the monster’s face distorted in horror. ‘I brought this with us from the shop. Thank you for reminding me.’

Blossom unleashed her furious fire on the puppet, instantly turning it to a cinder.

‘That was for Belle,’ said Fliss, treading the puppet’s ashes into the earth. ‘And for Corignis.’

With a final, desperate cry, the Mirk shrivelled to nothing. Without a host for its spirit, it was gone forever.

Archie dropped to his knees in exhaustion, and wiped the sweat from his brow.

‘You did it!’ said Fliss, running up to him and giving him a hug.

‘We did it,’ said Archie.

Billy kicked at the black fungus.

‘Thank goodness that’s over,’ he said. ‘I never want to eat another mushroom again.’