The Summer Queen

From back cover of Warner Books paperback December 1992:

MOON DAWNTREADER is Lady of Tiamat, pledged to forever end offworld exploitation and save the mers, whose blood gives immortality. But hostile Summer tribes and treacherous Winters plot to subvert... THE SUMMER QUEEN

INSPECTOR BZ GUNDHALINU, driven mad by his love for Moon, finds the ancient science of true starflight; but brings blood, greed, and ruin back to Tiamat when he returns to win... THE SUMMER QUEEN

REEDE KULLERVO, a dark genius with no memory or morals, his very soul burned from his body, is unleashed on Tiamat; a perfect monster programmed to destroy... THE SUMMER QUEEN

Ane when passion overwhelms the destinies of worlds, when betrayal devastates the tapestries of time, these three will be linked in a mystery that spans eternity, a threat to the entire cosmos, as enemies together, they must unlock the unbearable final secret of... THE SUMMER QUEEN

Synopsis from Open Library.