The Dread (Fallen Kings Cycle, 2)

Still reeling from plague and civil war, the Winter Kingdoms face an invasion
force from across the Northern Sea led by a dark spirit mage in the name
of ancient, vanquished gods. Summoner-King Martris Drayke and an
untested generation of new rulers must stand against an unholy alliance
of shadowy invaders. To prevail, Tris must win the uncertain support of
the Dread, spirit-beings that stand guard within the sacred barrows over
an even more fearsome threat imprisoned within. In the balance lies not
only the freedom of the Winter Kingdoms but Tris's life and the soul of
his young son, whose untested magic might be the biggest spoil of war.