In the book "Prince Caspian," which is the second novel in the series, Caspian blows Susan's horn to summon help during a dire situation. The horn has a magical quality that can bring aid to those in need. When Caspian blows the horn, the sound reaches the Pevensie siblings—Peter, Susan, Edmund, and Lucy—back in their world, drawing them back into Narnia for a new adventure.

The Pevensie siblings then find themselves transported back to Narnia to assist Caspian in his quest and face new challenges in the magical land. The horn plays a crucial role in connecting the worlds and bringing the characters together in the story.