Mardon, the son of King Nimrod, is a brilliant scientist whose good but misguided intentions eventually led him into evil in the service of Morgan le Fay. He first appears in The Eye of the Oracle.

Originally motivated by what was essentially the original discovery of genetic engineering, Mardon aided Morgan in the re-creation of the Nephilim and helped to grow the underborns. Recognizing the unusual intelligence and abilities of underborn Mara (not knowing at the time that she was an Oracle of Fire), he took her on as a protégé of sorts, acting as her mentor for a time. When he took her to the surface world to meet his father Nimrod, and she witnessed the amorality and corruption of the people there, she judged Mardon and his father's people to be evil; she ultimately rebelled against him and his work when the Tower of Babel was destroyed by dragons. Under the orders of Morgan, she unwillingly retaught Mardon the common language after Babel—enabling him to continue his work—until she escaped with Elam.

In Enoch's Ghost, Mardon worked in conjunction with his mother Semiramis to connect the worlds of Earth and Heaven—by building a new tower (undeniably hearkening back to his father's efforts at Babel) and attaching a supernatural cord of sorts to Bonnie Silver—which would enable the Nephilim and Watchers to overwhelm Earth. He also tried to force Ashley Stalworth, an anthrozil whose dragon heritage gave her superhuman intelligence, into helping him. This plot was foiled through the efforts of Elam and the sacrifice of Morgan's reformed sister Naamah.

At the end of The Bones of Makaidos, Mardon betrayed Acacia (Sapphira's sister and fellow Oracle of Fire) and brought about her death, unwittingly as the fulfillment of a prophecy.