As he manoeuvred to avoid another attack, his foot caught in a rabbit hole and he went down hard. The warrior poised to swing a sword towards Mallory’s head. In that instant, Mallory saw an opening. He managed to swing his sword in a short arc, hacking through the warrior’s left ankle.

The thing continued to attack as it fell, but it had been thrown off-balance. Even so, the flat of its sword still clipped Mallory’s temple, stunning him.

When he came around, Sophie was attempting to drag him up the slope. From further down the hill he could hear the sound of riders crashing through the undergrowth towards them.

Mallory lurched to his feet, his chest burning from the exertion of the fight. He tried to push Sophie towards the Blue Fire but her head was bowed, and when she looked up her eyes were crackling with sapphire energy.

‘It’s all right,’ she said. ‘This place is like a battery… I can draw on it…’

Mallory was unnerved by what he saw in her face — a primal fury about to be unleashed. Coruscating energy erupted out of her in a flash. When his vision cleared, there was a massive scorch mark across the slope and the warrior was nowhere to be seen. But from the trees the sounds of the riders drew closer, magnified as if their steeds were as big as rhinos.

‘Come on,’ Sophie said. ‘I’ve only bought us a few seconds.’

As Mallory and Sophie neared the whirl of Blue Fire, they heard the riders break from the tree line. Mallory turned to see three more of the hideous undead warriors, this time riding huge beasts that resembled horses but had scaly hides and cloven hooves.

Just as Mallory was preparing to make a last stand, the ground trembled and there was a sound like an avalanche as turf and soil tore back in a shower of earth and pebbles. In the gulf that opened up before them was revealed a majestic stone arch big enough to encompass four men standing on each other’s shoulders. Around it, the Blue Fire licked and sparked, but the centre of the arch was as black as space. Mallory had no time to contemplate it, for Sophie put one hand between his shoulder blades and shoved him into the gateway.