Nocturn are an enemy found exclusively within Spirit Tracks. They hide within the shadows of Dungeons, specifically the Tower of Spirits, and will chase and attack Link if he gets too close. They are virtually invisible within the dark, but become visible once a nearby torch is lit.
They can be defeated several ways: one way to defeat them is to use Link's Boomerang on a lit torch and then on a Nocturn. A second way is to have Princess Zelda take control of a Torch Phantom and have her attack a Nocturn herself. The final way requires the Bow of Light, in which Link is capable of shooting the enemy himself.