In the ancient past, Forneus was an alchemist that lived in the advanced city of Thabes. He was renowned and loved for his genius talents. At one point in his life, he was married to an unknown woman, whom he loved dearly. However, she eventually died under unknown circumstances, and since then, Forneus seemingly began to lose his sanity. He began to tirelessly work in his workshop in the deepest part of Thabes with two goals in mind: The first was to bring the dead back to life to use as subservient soldiers, and the second was to create a perfect lifeform. A mage worked with him at one point, but fled in fear when watching Forneus study a moving corpse.
While experimenting, Forneus accidentally created a new species of a shelled insect, which was about as large as a thumb, and resembled a shellfish. Naming these insects "thanatophages", Forneus discovered from his studies that they can assert control over a corpse like a parasite, move the body and strengthen the muscles greater than when the host is alive, and releases a preservation fluid that prevents the host from decaying. Thus, he constructed masks that resembled his late wife's face, and placed the thanatophagus in the reverse side of it. Created as a means to make the insects understand that he was their master, and that they served under his will, he dubbed those "Death Masks".
The alchemist then began to spirit away people that were near his workshop to experiment the thanatophages on them. The Senate began to suspect his involvement and sent messengers to urge him to come to a discussion with them. Forneus never replied, and the messengers never returned either. The Senate then sent forth soldiers, who ranked among the best in Thabes. Despite this, none of them ever returned. Meanwhile, Forneus was thankful of the Council, as because of them, he had many cadavers at his disposal, indicating that he had killed all the people sent and revived as his soldiers of the dead with the Death Masks. This eventually led to the Senate to label Forneus as demon alchemist and to be feared as one, before sealing his workshop permanently using the Sage's Shield.
Along with his research into the thanatophagus, eventually Forneus acquired the blood of a Divine Dragon, something he had taken a great risk to attain. With it, he mixed it with human fluids, some herbs and nectar, and stored it in a vial for 40 days. Eventually the concoction formed a "creature that resembled a human fetus, as small as a thumbnail".
Keeping the creature under the temperature of a horse's womb, Forneus studied it while giving it his blood every day. By the 80th day, the creature began to transform, its neck extending and growing a tail, while sprouting eyes. After some time, it resembled more like a dragon, albeit an usual shaped one, rather than the Divine Dragon it had been created from. When Forneus saw its six eyes, Forneus believed it to be smiling, and considered that it might have developed a consciousness. After the creature grew to be as large as a puppy, Forneus caught glimpses of the terrifying power it had. He sought to try and find a way to control its rampage before it would reach adulthood, but was unable to find one.
Eventually, Forneus began to hear the creature's voice in his head, and felt both were able to see inside one another. Forneus saw the creature's dark and violent thoughts, such that he felt himself tremble that the creature saw and seemed to smile at. Fearful of how he created something that was beyond his control, he sought to try and kill it despite how painful it was. It is unknown exactly what occurred afterwards, but it is likely that Forneus was killed by the creature for his actions.
Originally, Forneus was simply a genius alchemist in Thabes. Loving his wife a great deal, her death was the catalyst for his descent to madness, being clearly expressed when the very Death Masks that he constructs to hold the thanatophages are molded to resemble his wife's face.
However, due to said insanity, Forneus developed an obsessive desire for control, seeking to control not only the dead, but also the perfect lifeform he had created, with the Death Masks he created specifically to make the insects understand that they were controlled by his will. Alongside that, he also showed little care for life following his insanity, as he actively captured humans to use his Death Masks on, and had all messengers and soldiers sent in after him killed as well, being in fact thankful to the Senate for giving him an abundance of corpses. It also led to him having a twisted sense of caring, as his army of masks, though would be called hideous by others, were considered adorable pets to him.
Such desire for control ultimately led to his death, as due to how he was incapable of controlling the power within his "perfect lifeform", to the point that Forneus feared it himself, Forneus sought to kill his creature, though he confessed that it would pain him as well.
In demonology, the study of demons, Forneus is a Great Marquis of Hell, and has twenty-nine legions of demons under his rule. This can be attributed to the birth of the Risen. He is depicted as a sea monster, and one could argue Grima resembles such.