Serpent's Kiss by Thea Harrison — book slip

In the third novel of the Elder Races series, Wyr sentinel Rune Ainissesthai makes a bargain with Vampyre Queen Carling to save his friend's life, without knowing what she would ask from him in return.

When Rune attempts to fulfill his debt, he finds Carling's power becoming erratic, causing her followers to flee in fear.

Despite the danger, Rune is drawn to the ailing Queen and decides to help her find a cure for the serpent's kiss—the vampyric disease that's killing her.

As their desire for each other escalates alongside Carling's instability, the sentinel and the Queen must rely on each other to survive.