The Fragile Threads of Power by V.E. Schwab

Seven years have passed since the doors between the worlds were sealed. Seven years since Kell, Lila and Holland stood against Osaron, a desperate battle that saved the worlds of Red, Grey and White London. Seven years since Kell’s magic was shattered, and Holland lost his life.

Now Rhy Maresh rules Red London with his new family – his queen, Nadiya, their daughter Ren, and his consort, Alucard. But his city boils with conspiracy and rebellion, fuelled by rumours he is causing magic to fade from the word.