Power of the Serpent The Seed Bearers Trilogy #2 Peter Valentine Timlett 1987
Druthin, Arch-Mage of the Druid cult, called his priests to the sword when Mog Agorn defected to the demon powers and besieged sacred Stonehenge. Not only was the survival of Druidry at stake, but the fate of humanity as well.
For Mog Agorn had made an awesome pact with the dark forces. He'd bedded his only daughter to spawn a vile champion from the depths of hell. And he'd sworn mankind's unborn children to the evil ones in return for victory.