Chthonic

The Earth is not a foundation. It is a cage. Two kilometers beneath the Harz Mountains, a commercial drilling operation breaches a structure that shouldn't exist: a temple of seamless black stone, sealed before the dawn of human history. For Dr. Elias Thorne, a skeptic and linguist, the discovery is an archaeological miracle-until he matches the inscriptions to the Al-Azif, a cursed medieval scroll long dismissed as a hoax.
Elias realizes too late that the drill didn't just hit a cavern; it broke a lock. Now, a corrupt cabal of Europe's elite is racing to harness the power rising from the deep, while a violet fog spreads across the continent, warping the laws of physics and biology. As ancient predators wake to reclaim the surface, Elias and historian Lena Vogel must decipher the Scroll's counter-ritual before the world is swallowed by the dark.
But they soon discover that to close the gates of Hell, one does not simply turn a key-one must become it.