Here's the description of The Heir of Night from the back cover of the Advance Readers Edition:
IF NIGHT FALLS, ALL FALL...
In the far north of the world of Haarth lies the bitter mountain range known as the Wall of Night. Garrisoned by the Nine Houses of the Derai, the Wall is the final bastion between the peoples of Haarth and the Swarm of Dark – which the Derai have been fighting across worlds and time.
Malian, Heir to the House of Night, knows the history of her people: the unending war with the Darkswarm; the legendary heroes, blazing with long-lost power; the internal strife that has fractured the Derai's former strength. But now the Darkswarm is rising again, and Malian's destiny as Heir of Night is bound inextricably to both ancient legend and any future the Derai – or Haarth – may have.