River Runs Red — Jeff Mariotte

Three friends return to their Texas hometown, and a supernatural war that will decide the fate of worlds, in this horror thriller.

As teenagers, Molly, Byrd, and Wade faced inconceivable evil in an underground labyrinth on the banks of the Rio Grande. Now they are reunited as adults, about to discover that their terrifying experience was only the beginning.
Something has drawn the three friends back to their small Texas town and the caves in which they faced their fate. A mysterious force is plunging them into a supernatural war that spans across the globe, through raging rivers, mysterious murders, long-buried gods, and secrets worth dying -- or killing -- for.