The Dark Mistress — W.C. Hawkens

In this last book of The Dark Raven Trilogy the professor is poisoned, several members of the Elder Council of Witches are mysteriously murdered, and a close friend is banished to another dimension. To make matters worse Serena, her sister, Lady Marna and the professor must face their deadliest nemesis, a cold ruthless leader of an unscrupulous cult determined to reclaim an ancient ring that bestows its possessor with the power and immortality of a god.