He was genteel and well-spoken, but took pleasure in mocking others and could be driven to fits of blind, unreasoning jealousy. He wielded a magical war pick known as Ironfang, one of the four elemental tools made by Vizeran DeVir.
His backstory is revealed in the adventure book, Princes of the Apocalypse.
Marlos was born into a wealthy Cormyrean family of Waterdhavian, but squandered his family's wealth on indulgence and luxury. His estate became known for the most depraved forms of entertainment, as well as outrageous masquerades—Marlos's vanity led him to require all guests to don hideous masks to ensure that none would be more beautiful than himself.
As he got older, he sought to preserve his youth and looks with magic, eventually finding a ring containing the stolen vitality of a fey creature. So long as he wore the ring, his imperfections vanished and none could resist his charms. One day, however, the ring suddenly transformed Marlos into a medusa. Driven mad by his fate, he fled his family's home with his mask collection.
Marlos wandered from one ruined estate to another for several years, until he began to receive visions leading him to a ruined monastery within the Sumber Hills. There, a hidden stairway led him to a cavern in which he found a magical war pick named Ironfang imbued with the power of the primordial Ogrémoch, Prince of Evil Earth. When he claimed it, he saw his purpose with sudden clarity. Becoming a devout follower of Ogrémoch, he began gathering followers into the Cult of the Black Earth.
All their work culminated in 1491 DR at in the Fane of the Eye, under the ruined dwarven city of Tyar-Besil. Working simultaneously with the other three elemental cults, the earth cultists tried to summon Ogrémoch to Toril using members of a missing diplomatic delegation from Mirabar and other captives as sacrifices. However, they needed a fortress.
As the Helm's Hold returned to peace after a long time oppression by demonic forces, Marlos seized the chance and disguised his cult as the Black Earth Mercenary Company. He falsely offered to help defending the reclaimed Helm's Hold. Though the council was wary, Lord Neverember vouched for their leader. Once the god Helm returned, however, the mercenaries revealed themselves as the Cult of the Black Earth, and Marlos himself was revealed to be a decadent medusa. As the cultists turned against the people they had been hired to defend, the council holed up within Helm's cathedral, the one place the cultists seemed unwilling to go, while outside a towering stone fortress known as Reclamation Rock erupted from the ground.
However, the problem soon comes to known in Neverwinter. When Archdruid Morningdawn of the Emerald Enclave arrived in Neverwinter with word of a new threat rising in the Sword Coast, Lord Neverember took it seriously. With his permission, she erected the Tree of Elemental Balance to guard the city of Neverwinter. She explained that the currents of elemental forces had changed in such a way to cause dangerous concentrations of the elements, and that the tree would allow her to stabilize the flow. However, someone or something was interfering.
The Emerald Enclave had reporting a building pressure of earth and water in the Ebon Downs, and while the great warrior Minsc had volunteered to investigate Morningdawn believed that more than his zeal would be needed and asked the Heroes of Sleeping Dragon Bridge to go with him. Deep in a cave within the Ebon Downs, Minsc and the heroes found cultists of elemental earth and water creating concentrated manifestations of elemental energy known as elemental seeds. When they brought the earth and water seeds back to Morningdawn, she concluded that the disturbance she felt was due to these elemental cults, who were apparently trying to create elemental nodes - two-way conduits to one the elemental planes. Later, Minsc and the adventurers foil the fire and air culitists' plan in Pirates' Skyhold and received the fire and air seed.
Fraunstrated for their failure, the four prophets gathered their army and attacked Neverwinter in an attempt to destroy the Tree of Elemental Balance. Marlos' army arrived after the water cultists, which included earth cultists, stone monsters and earth archons. After their defeat, the prophets retreated back to their respective fortresses to bulid elemental nodes. Meanwhile, Archdruid Morningdawn tasked the heroes with eradicating the cults' presence along the Sword Coast.
When Lord Neverember learned of what had happened beside Helm's Hold, he sent Neverwinter's ranger corps along with the Heroes of Sleeping Dragon Bridge to reclaim Helm's Hold, finding that the cult was corrupting the loyalists of Helm and making them into earth culitist, starting a tyranny not less horrid than Rohini the Prophet's reign. After securing first the Watcher's Forest and then the city itself, they entered the earth node that Urnrayle had created deep below the earth and struck down the prophet himself before he could succeed in making a devastation orb.