Defender's Vow. Defenders of the Forbidden City, #6

The Forbidden City was never meant to be entered. It was meant to keep something in. For centuries, ancient seals have held back prisoners and powers too dangerous to walk free. The Defenders have one duty: guard the city and make sure the horrors buried within never escape. But the seals are weakening. Wardens and dragon riders are vanishing. Shadowbound creatures are spreading, and someone is determined to tear apart protections that have endured for generations.
From a wayward prince searching for his missing sisters to the Defenders patrolling the city's crumbling boundaries, each new threat reveals a danger far greater than anyone imagined. Corruption is spreading beyond the prison walls, old enemies are waking, and those sworn to protect the realms must decide how much they're willing to sacrifice. Some prisons exist for a reason. If the Forbidden City falls, what escapes may be impossible to stop.
Enter the complete five-book Defenders of the Forbidden City series: an epic fantasy adventure of ancient magic, found family, fierce defenders, and battles that could decide the fate of more than one world.