Shadow on the Stones by Moyra Caldecott

Shadow on the Stones is the third book of the Guardians of the Tall Stones series.

Kyra, priest of the Temple of the Sun, and her husband the Lord Khu-ren, have guided their civilisation to a time of spiritual strength, psychic energy, and communal peace. But there is a shadow on the Stones -- the spreading influence of the terrible god Groth, dark god of chaos and barbarity. Only the priests of the Temple possess the power to resist the tides of destruction...