What you are looking for is "The books of the Kingdoms" trilogy by Angus Wells.
Blurb text from he back-covers of the books:
The First book of the Kingdoms:
WRATH of ASHAR
HE WAS BORN IN THE FLAMES OF A FIRE BEYOND
IMAGINING, UNTOUCHED AS ALL AROUND HIM
THE GREAT FOREST BLACKENED AND SHRIVELLED
IN THE FLAMES...TAWS THE MERCILESS!
It is a time of prophecies fulfilled, as a mysterious being with
terrifying powers comes forth from an inferno to unite the
warring barbarian tribes of the North against the Three
Kingdoms: Tamur, Kesh and Ust-Galich. Hope for the
Kingdoms lies in the birth of Kedryn Caitin, whom the Holy
Sisterhood of Kyrie believe to be the champion whose
coming is foretold in their sacred book. But Taws also
knows of the boy's existence, and as his armies cut a bloody
swath across the peaceful land , Kedryn and his kin come
ever nearer his merciless grasp...
Sphere Books Limited, 1988
The Second book of the Kingdoms:
THE BATTLE WAS OVER. KEDRYN CAITIN HAD LED
THE THREE KINGDOMS TO VICTORY OVER THE
BARABRIAN HORDES. IT SHOULD HAVE BEEN A
TIME OF PEACY….
But Taws was not defeated. He who was spawned in the cold
fires of Ashar swore vengeance that with his dark sorceries
the Three Kingdoms would fall.
As for Kedryn, the Chosen prophesied in the sacred book of
the Sisterhood, he was blind. His sight had been stolen by a
bewitched sword. He must seek the shamans of the
Barbarians. Only with their help cold he enter the
netherworld to confront the shade of the warrior who in life
had wielded the blade.
While Kedryn travelled far, accompanied only by his loyal
friend and a holy Sister, Taws the Messenger saw his
opportunity. With the Chosen absent, he would use his
terrifying magicks to gain the throne of the Kingdoms by one
dark means or another…
Sphere Books Limited, 1989
The Third book of the Kingdoms:
THE WAY BENEATH
TAWS THE MESSENGER IS DEFEATED AND THE
THREE KINGDOMS AT LAST KNOW PEACE. KEDRYN
CAITIN, THE CHOSEN OF KYRIE, IS HAILED HIGH
KING. AT HIS SIDE HIS NEW-WON BRIDE, WYNETT.
BUT DEEP WITHIN THE REALMS OF THE
NETHERWORLD THE GOD, ASHAR, PLANS A
All Kedryn has fought so hard to build stands in jeopardy
when the Lord of Fire unleashes leviathan on the world to
take Wynett for is own, living bait in a demonic trap.
To win back his love Kedryn must enter the world of
ghosts, to claim the hell-forged blade wielded by the long-
dead Drul, ancient overlord of the barbarian hordes. Only
that dreadful sword may prevail against the awful power
of the vengeance-bent god, and to save Wynett from
damnation eternal Kedryn must brave the perils of the
god's domain — must face Ashar himself in battle for his
Sphere Books Limited, 1990