An Ember-Grub is about as long as a finger and as thick as a quill, soft, ringed, and blind, the grey of the ash it lives in. A grub that has fed shows a faint dull red through its skin, like a coal seen through a layer of cinder, and the glow fades as it digests. It has no legs; it moves by drawing its rings together and pushing through loose ash, slowly, as everything on the plane moves. It makes no sound a person can hear. A handful smells of burnt wood and nothing else.

They live in the ash drifts and the cracked ground under them across the plane, a few to a stride in open ash and in hundreds beneath any lasting source of warmth: the hearth stones of an Ash-Walker hold, a buried warm stone, the place where a traveller sleeps two nights running.

Ember-Grubs feed on heat and on nothing else. A grub senses warmth through the ash and works towards it, and others follow the first, so that by morning a cluster lies packed under a fire or under a sleeper’s bedroll. They do not bite or burrow into flesh. When the heat above them is gone they disperse into the ash again, each carrying its faint red glow away until it fades.

A cluster of Ember-Grubs drains heat faster than the plane’s own decay. A fire banked over grubs dies in a fraction of the time it would otherwise last, and a hot stone left on the ash overnight is cold by morning. A crushed grub gives back a moment of its stolen warmth, a brief prickle of heat on the skin.

They hatch in the ash from eggs laid wherever a cluster has fed well, and grow slowly, as all life on the plane does. A grub that finds no warmth for long enough goes grey through and through and crumbles into the ash it came from.

To the Ash-Walkers, who hoard heat with religious care, the grubs are thieves: they sift the ash around their hoards and crush every grub they find, and a hold that lets them in can lose a winter’s stored warmth. Travellers learn to sleep on stone or on layered hides rather than on bare ash, and to move a fire every night. The grubs cannot kill on their own, but on a plane where heat is life a night spent over a cluster can leave a traveller too cold to rise.

Villages near a Cold Gate on the Material Plane blame "ash-grubs" when a hearth goes cold overnight for no reason anyone can find, and sweep the hearth with a bundle of green twigs to drive them back through the gate. Whether any grub has ever come through is not known; the hearths that go cold are most often only badly banked. Some Ash-Walkers are said to keep a few grubs in a sealed pot, fed on scraps of heat, and to eat them crushed in the worst of the cold for their moment of warmth; travellers' accounts disagree on whether this is true (see Knowledge of the Cosmos).