City Of Magic - Elfblood Trilogy, #3

Charlie has finally reached the City Of Magic, where waits the lost elven magic he has been searching for. But the city is a ruin, his friends are missing, and there is a darkness known only as the Void slowly creeping over everything. When Charlie comes across a group of elves, he hopes they will be able to help him locate the Silver Catacombs. But everything is not as it seems. There are two groups of elves in the city, one who resists the Void and one who worships it.
It'll be up to Charlie to figure out who his true allies are. Danger lurks around every crumbling ruin and betrayal always comes from within.