Trolls usually live under bridges and pop out at people passing at midnight when they're strong enough to eat them. Tanith Low fought the troll beneath Westminster Bridge in London in the first book and succeeded. They are ugly little things with almost no hair and sharp yellow teeth. At midnight, when they're at their strongest, they grow to twice the size of a human. They only have one name which is their True Name. Caelan mentioned Trolls in Just Another Friday Night.