Dark Fire (The Last Dragon Chronicles #5) by Chris d'Lacey

The universe is threatened by dark fire...

"Dragons are hardly new," David said. "This is their world. It always was. When everything is ready, they will colonise again. Only this time, things will be different."

With the Earth on the brink of recolonisation by dragons, David Rain is sent on an important mission: to seek out and destroy a trace of dark fire, the deadliest force in the universe. But with success could come a terrible price, the sacrifice of a beloved clay dragon. How much does the life of one small dragon count, when the alternative is the birth of a darkling...?