Sphere of Influence (Chronicles of the Custodians #3) by Martin Middleton

She is the Darkness....She is the underlying evil which has lain upon the world for an eternity, waiting for the first touch of life so She could corrupt it, twist it to Her ways....She has decided to use her powers to start a war which neither side can win.   And after this war, She will claim the pitiful survivors as Her own.

Teal the Death Lord, hero of Circle of Light and Triad of Darkness, has banished the Dark Gods from his world and secured an uneasy peace in the lands under his dominion.   But the cost has been great -- the Gods of Light have been forced to follow the Gods of Darkness into oblivion. If Teal is to return the Light to his lands, he must also unleash the forces of Darkness and once again do battle with evil.

Teal and his companions set out on a perilous quest for the source of Darkness.   It is the source that Teal must destroy if a time of mass destruction and unparalleled bloodshed is to be averted...

In an awesome confrontation between good and evil, Teal comes face to face with his destiny...and must make the ultimate sacrifice if he is to curb the Darkness's ever-expanding sphere of influence.