While Stig watches the fog, I walk around the cabin to the garden. Not that you can call it a garden – more a few acres of grass less tall than the surrounding heath. I often saw Mormor scrubbing dirt from her nails at the kitchen sink, yet there are no flower beds or vegetable patches. Once when I woke early, she was on her knees weeding around the roots of the twisted tree.

I pause before it now, my heart pounding. It stands on its own grassy mound, like it was planted there deliberately. Three times the height of the cabin, its enormous grey trunk is a mass of bulges and knots, and so big it would take seven of me with arms outstretched to encircle it. Thick green moss coats the base of its trunk: a plush velvet skirt covering the rough, scaly bark. The wind has died to nothing and for once its mighty branches barely stir.

It looks so different to when I came in summer. Menacing almost. Keeping my distance, I walk around it, stepping over knobbly roots that protrude from the earth like the veins on a hand. One of the huge roots contains a deep pool of water. Mormor said it holds a natural spring, and that’s why it never dries up.

Where is that dog? ‘Gandalf!’ I scan the dark edge of the forest. The fir trees crowd together like soldiers in battle formation, their trunks forming a hard drawn line. We should have kept Gandalf on a long rope. What if he’s wandered off and there really is a wolf?

I trudge through orange spiky bracken, shouting now.

‘Hey, wait!’ Stig yells behind me.

I turn to see a looming mountain of fog. It smothers the light, giving the world an eerie feel. Water glistens in Stig’s hair, like dew on a spiderweb.

‘Gandalf – where exactly did you see him last?’ I call, my voice breathless.

Stig points. ‘He was digging there, by the tree.’

I clamber back over the bracken. ‘And you didn’t call him back?’

Stig’s eyes are wide with worry.

‘Gandalf!’ we shout together.

‘Is that him?’ I point at the twisted tree. ‘There! I thought I saw something move.’

We jog to the tree then stop, both at the same time, as if we’ve hit an invisible wall.

Inside its huge gnarled trunk are several hollow chambers, formed by the weird way the tree has grown. As a child I used to love playing in the dark spaces, but I would often have bad dreams about them too. Sometimes I think the tree has always been on the edge of my nightmares, waiting to trip my feet and snatch at my hair.