According to the writings in The Lesser Key of Solomon, Forneus is the thirtieth spirit listed in the Goetia. He is a Great Marquis of Hell with twenty-nine legions under his command, partly comprised of the order of Angels and Thrones, and appears as a sea monster. When summoned, he can make men well-versed in rhetoric, give him a good name, teach him foreign tongues, and make him trusted by friend and foe alike.
"Origin: Israel. One of the 72 demons of the Goetia. He was one of the Thrones who fell from grace. He appears as a silver sea monster, but can take the form of a human as well. He grants knowledge of various arts and sciences to hims summoner. He can also change the hatred of one's enemies to something like friendship."
"One of the 72 demons of the Goetia. He appears as a great sea monster. He is skilled in languages and is a master of rhetoric."
—Shin Megami Tensei III: Nocturne compendium
Forneus is the first boss, fought in the Hospital when the Demi-fiend reaches the exit. A Decarabia in Shibuya claims to be Forneus's friend and is annoyed by the latter's supposed tardiness, although there is no given explanation for their friendship. Some suggest that it is because both appear as creatures of the sea, others that it is simply because they are both Marquis of Hell, although this is still only conjecture.
Forneus is the boss of the Fourth Sphere of the Womb of Grief. He swears to avenge Amon by fighting Alex, but chooses to settle with the protagonist when he makes it to Forneus before she can.
Forneus's attacks are mainly physical and ice-based. His signature Death's Bite hits random targets 4 times, and he telegraphs this by using Charge. Once he hits half health, he unleashes Abyssal Prison, which is a field effect that lets him auto-cast Abyssal Scream at the end of every turn for 3 turns, inflicting Fear across the party.
Despite all this, he shouldn't be so tough. At this point, if one has been diligent with gathering D-Sources, Luster Candy and Debilitate should be easily acquired. His weakness to Gun skills makes it easy for the protagonist to constantly exploit his weakness. Fight conservatively, though, as the player gets into another boss fight immediately afterwards.
After his defeat, Forneus surrenders the Fourth Fruit. He also becomes available for fusion and unlocks the way forward. Forneus's fusion requires Decarabia, Kikuri-Hime, and Sui-Ki. He leaves behind the Spotted Leather forma, which is used to create the Sub-App Co-Op Enhance C, which allows Co-Op attacks to trigger on critical hits.
Shortly after he retreats, though, Alex appears and attacks the protagonist.
"One of the Goetia's 72 Demons, he appears as a great sea monster. He is linguistically skilled, and a master of rhetoric."
—Persona 3 compendium
Forneus can be obtained via fusion, or through Shuffle Time in the second block of Tartarus, Arqa. The special non-mythological relationship with Decarabia is rendered again in this game, in which these two personas are able to perform a combo skill called Best Friends.
"One of the 72 demons of the Goetia, he appears as a great sea monster. Known as the great Marquis of Hell, he is skilled linguistically, and is a master of rhetoric."
—Persona 5 Compendium
Forneus appears as an obtainable Persona. Forneus also appears as the second boss of Shido's Palace, and is the manifestation of one of Shido's allies, a Former Noble. As a Shadow, it's known as the "Rhetorician of the Sea."
"One of the 72 demons of the Goetia. When summoned it appears as a large winged fish or similar sea creature. This creature is especially well veersed, teaching its summoner the art of rhetoric. One who masters Foreus' craft is able to convince even the most stubborn minds."
Kazuma Kaneko made his design look like a manta ray because the manta ray are sometimes known as "devil fish;" Forneus appears as a sea monster. The demon's skeletal structure, namely its spine and forelimbs, also bear a distinct resemblance to a human body.