The Science of Power — Ru Emerson

WOMEN OF CRAFT AND COURAGE...
...at great risk to herself, must call on all of her extraordinary gifts to combat a traitorous evil threatening to poison all that is decent in the land of Rhadaz. Under the cold-blooded rule of an emperor ,one mad, she must halt the influences of outer worlds schooled in the art of death and trained to worship...

Though her nephew Chris and his reluctant bride have finally found the source of the dread drug Zero, though her sister Robyn has found a measure of happiness with Duke Aletto, Jennifer cannot rest. For everyone she holds dear is bound for one last great battle, a battle where the power of the sword will be matched against the might of dark magic...