Marduk Kurios is a high level demon and ruler of one realm of hell. Like many other demons, he has falsely taken on the titles of Satan, Lucifer and The Devil in order to further strengthen his power and devotion. In ancient times the Sumerians worshiped Marduk after seeing a previous battle his legions of hell took part in on Earth. Intrigued by this new worship he kept track of the mortal realm over the centuries. He became enamoured by a family of satanists called Hellstrom and took the shape of one of them. In this guise he married the mortal Victoria Wingate and soon after moved to Lake Fire, Massachusetts. There Victoria gave birth to Daimon Hellstrom, and a year and a half later gave birth again, this time to Satana Hellstrom. Over the years Marduk could see that his son did not care for his dark family legacy and instead lavished affection to his daughter who did. While performing a ritual with his daughter involving animal sacrifice, Victoria accidentally walked in and discovered Marduk's true nature. Driven insane, Victoria spent the remainder of her life in an asylum where she kept a diary she meant to give to Daimon once he was old enough to understand.
Marduk took Satana with him to his hell realm while he left Daimon on Earth to be cared for by servants. Upon his 21st birthday Marduk invited his son to rule by his side, offering eternal life and power. Daimon rejected his father's wishes and stole Marduk's Netheranium trident, taking with him a portion of Marduk's power. Thus the rivalry between father and son began.
Marduk then tested Satana's loyalty by having her battle four sorcerers. Killing them she discovered that her father was behind the plot. When she was confronted by him Satana failed to kill one of her new allies in honor of her father. This angered Marduk and he banished Satana to remain on Earth until her first mortal death.
In a battle with the Defenders, Daimon's evil soul was released and he agreed to serve his father in hell. During Daimon's final training Marduk asked Daimon to kill a human in his honor. When Daimon failed to comply Marduk revealed that although he has lived under many faces over the centuries that not all of them have been evil. He told Daimon that he was secretly pleased with his choice but then banished him to Earth, no longer under his care.
Daimon eventually learned his father's true name from the witch Lavoisin. When next he confronted his father he used this name and received a black halo over his head and became the new ruler of his father's realm in hell. Greatly reduced in power, Marduk tricked the male witch Andrew Kale into opening the Tome of Zhered-Na. In doing so the demon Hellphyr was released. The Hellphyr then began to attack various magical persons in order to steal their power and kill them shortly after. In causing this, Marduk had hoped to rid the world of a good percentage of its magic users in order for him to acquire a better rank among magic's stairway. Marduk offered Satana the opportunity to help him in this scheme and thus receive a higher standing herself in the realm of magic. Refusing his offer, Satana killed the Hellphyr with the help of her fellow witches Jennifer Kale and Topaz. In doing so Marduk made enemies of both his children as he searches for another way to regain his previous level of power.
Despite the continuing conflict he has with his children, Marduk still receives offerings of human souls from his daughter Satana. For every nine souls she devours, she offers her tenth victim's soul to her father.
Marduk was killed by the Chaos King during the Chaos War, his realm assimilated into the Chaos King's being. He was presumably revived when Hercules used Gaea's power to restore all that the Chaos King had destroyed.
Marduk more than many other demons claims to be the true "Satan" or "Devil" which may have been an actual entity but also refers to the absolute ruler of evil. Currently, no hell-lord has amassed enough power to rightfully claim that title.
Marduk at one point expressed disbelief is the entity of the "true Satan" even existed.