Keepers of the Underdeep

Black Hollow has always kept its secrets buried - literally. Beneath its rain-soaked streets lie forgotten tunnels, flooded canals, and a civilization no one above ever chose to believe in.
Sixteen-year-old Kai Mercer just wanted to get through chemistry class. Instead, a pull he can't explain drags him into the dark beneath the city, where a dying stranger presses an ancient Ring into his hand - and dies calling him something impossible: a Keeper.
For centuries, Keepers have held the Balance, the fragile law keeping humanity's world separate from the Underdeep below and the horrors sealed deeper still. No Keeper has ever been human. No Ring has ever chosen someone this unprepared. And now something ancient, buried far beneath even the Underdeep, has felt the Ring change hands - and opened its eyes.
As Kai's ordinary life collapses into one of monsters, memory-eating Echoes, and a power that consumes everyone who wields it, he learns the truth is worse than an invasion: the Ring was never just protecting the Balance. It's been straining it all along. And the one form powerful enough to fix that - the Sixth Form - has never been awakened, because no one has ever been human enough to reach it.
With his best friend refusing to leave his side, a girl who sees the memories hidden in broken things, and a demon whose loyalties no one can name, Kai has to decide what he's willing to trade - memory, feeling, time, himself - to hold a broken world together.
Some inheritances aren't a gift. They're a countdown.