Those Who Went Remain There Still — Cherie Priest

Heaster Wharton is dead, and his passing might mean an end to hostilities between the Manders and the Coys. If the the elderly patriarch showed the kindness and foresight to split his land cleanly between his feuding descendants, then a truce could be arranged.
But his final request is a strange one, delivered across the country to the straggling remnants of his tribe. Representatives from both families must visit a cave at the edge of his property in the hills of Kentucky. There, he promised, they would find his last will and testament.
But there's more than paperwork waiting underground, as vindictive old Heaster was well aware.
In 1775, Daniel Boone and a band of axe-wielding frontiersmen struggled to clear a path through the Cumberland Gap into the heart of Bluegrass country, and they did not work unopposed. Hounded and harried by an astonishing monster, the axe-men overcame the beast by sheer numbers and steel. They threw its body into a nearby cave.
It was not dead.
And now, it is not alone.
Crippled and outraged, for 100 years something terrible has huddled underground, dreaming of meat and revenge. But its newest callers are heavily armed, skeptical of their instructions, and predisposed to violence.

With their guns and their savage instincts, Heaster's grandchildren will not make for easy pickings.