Why Weeps the Brogan?

In this futuristic tale set in a post-apocalyptic world of terrifying spiders two children face a creature, hideous and unnatural, the Brogan. Despite their terror they feel the need to feed the creature every day. But where are Gilbert and Saxon? How long they've been there? Are they the only humans left in the world? What does the Brogan want? And what exactly happened in the past that they cannot clearly remember.