The Lair of Bones: The Runelords, Book 4

Prince Gaborn, the Earth King, has defeated the forces arrayed against him each time before: the magical and human forces marshaled by Raj Ahten, who seeks immortality at any cost and has given up his humanity in trade; and the inhuman, innumerable, insectile hordes of the giant Reavers from under the Earth, whose unknowable motives are inimical to human life.

Now there must be final confrontations, both on the field of battle, with the supernatural creature that Raj Ahten has become, and underground, in the cavernous homeland of the Reavers, where the sorcerous One True Master who rules them all lies in wait - in the Lair of Bones. The survival of the human race on Earth is at stake.