Tokoroth are demons trapped in the bodies of children, used by Soul-Eaters to carry out things such as spying, poisoning food or harming people the Soul-Eaters themselves can't reach.
In several of the books, clans complain about stolen children (in Oath Breaker, Torak is actually blamed for this). However, it is the Soul-Eaters creating tokoroth. They turn a child into a tokoroth by keeping it in darkness and isolation for many years, until it has "forgotten the touch of its mother's hand, forgotten its very name". After it is "nothing but an empty husk", the Soul-Eater summons a demon and traps it inside the child.
Although tokoroth are really just children, their teeth and nails are sharpened to fangs and claws. They seem to acquire strength and agility from the demons inside them, too, because tokoroth do things Torak can't to get away from him in several of the books.