The Demon King’s Heart

A monster should not have a human heart. But when Kyle Arden's chest begins to beat with the pulse of the Demon King, saving the world may mean becoming the thing everyone fears most. Kyle is an ordinary young man from Grayhail's lower district, fighting for one thing: keeping his sick little sister, Nia, alive. He has no noble blood, no holy blessing, and no reason to believe the world will ever protect people like them.
Then a festival night turns into a massacre. An abyssal beast erupts from beneath the city. Knights fall. Holy seals shatter. The crowd runs. And Kyle, desperate to save Nia, awakens a black flame that should belong only to legends, monsters, and kings who were never meant to return. Now the Church wants him chained. The soldiers want him controlled. The people whisper that he is a demon in human skin.
Inside his chest, seven ancient seals hold back a power that answers to fear, rage, hunger-and the enemies crawling out of the abyss. But Kyle cannot simply run. Nia's life may depend on the same cursed heart that is slowly changing him. The black flame can destroy monsters no one else can touch. And somewhere beneath the lies of the Holy Ring Church, the sealed history of a broken king begins to stir.
To protect the only family he has left, Kyle must enter a world of forbidden magic, military trials, abyssal creatures, holy conspiracies, and dark power that grows stronger every time he survives. If the Demon King's heart opens fully, Kyle may save humanity. Or he may become the nightmare that ends it.