The Warrior is a Gnawer?

You heard this story before, about a boy who fell into underground world with his sister. A world filled with giant bats, rats and cockroaches..the humans that lived there had a prophecy about him..calling him the Warrior..how he gave them time and life..how he made many friends..but lost as well..this is that story..but with one differnce Instead of a human, he was a giant rat