Semiramis is the wife of King Nimrod and mother of Mardon. She first appears in Enoch's Ghost, where she tries to tempt Elam off his path in the Bridgelands. She is a shady character whose intentions are not originally clear. She also appears in Second Eden, giving Billy Bannister information about the "dragon" Arramos, which is true, but intended to lead Billy, Elam, and the villagers into a wrong course of action. Ultimately she is revealed to be plotting with her son to connect Hades, Second Eden, and Earth, and it is her supernatural "rope" which allows this plan to come to near fruition. Ultimately, she and Mardon are defeated, but the readers are never told their actual fate.
By the time she appears in the books, Semiramis is actually long dead, but uses an apparent ability to travel between dimensions to interact with the living worlds.
Children of the Bard
In Children of the Bard, she continues to scheme and deceive. In the Seventh Door, she disguises as Matt's former foster sister Darcy, and ends up meeting her ultimate destiny in the Lake of Fire, when Matt finds his real former foster sister in a barn (The place she ,(Semiramis), meets her ultimate demise).
She dies fully in the Seventh Door from the Children of the Bard series. Her death is caused by Matt Banister tying her up to a nuclear rocket designed by her son, Mardon.