The Gathering Of The Lost by Helen Lowe

Tarathan of Ar and Jehane Mor, ride into the great city of Ij in time for it's grand Festival of Masks. But soon after their arrival they witness a terrible slaughter as their fellow heralds are targeted an assassinated. They must flee for their lives across the city as they discover Swarm agents at work as they attempt to destabilise the entire River Cities network for their own ends. And five years after her great flight from the Derai Wall, Malian remains hidden to those who seek her. But she has not been idle. Her goal is to muster all Derai magic users that have fled into exile rather than face destruction. Only by uniting against the Swarm menace can they hold their own against the dark tide and she has hunted down every rumour of their presence. And she has developed her own powers that the Swarm must learn to respect ? and to fear. For Malian won't see her people fall to a dark tide of twisted magic as demonic forces subvert a way of life.

This book is part of the "The Wall of Night" series.
Here are some other books from this series: