This cocoon is an item owned by Coulter. He says that there is a certain species of pixie in Norway that hibernate in these cocoons in the winter. These cocoons can become very valuable if properly treated. Biting on the cocoon instantly expends it, enveloping the biter. It becomes an impregnable shelter and has enough food in the walls to keep one fed for months. Coulter gives this to Seth to save him from the Revenant. It also helps him survive being eaten by Olloch, the demon
It looks like a cocoon. It's small enough to fit in someone's pocket.