Orcus was a god of the underworld, punisher of broken oaths in Roman mythology. He was more equivalent to the Roman Pluto than to the Greek Hades. He was portrayed in paintings in Etruscan tombs as a hairy, bearded giant. Orcus is also said to have inspired orcs, a race of pig-like men who frequently appear in modern fantasy. This is apparent in the series as Orcus appears as a grotesque pig-headed creature.
"Origin: Italy. A malevolent Roman god. He is one of the gods of the underworld, and has a pig's head and bat wings. He's daring but violent, and will massacre enemy troops in order to eat them. He sometimes appears as a giant dragon with fangs of iron."
Orcus is the subject of the challenge quest The Secret to Durable Clothing. He can be found by heading north of Ueno and taking the river crossing across to the Minami Senju region. Despite the quest description, Orcus can rarely be encountered throughout the desert region west of the Minami Senju area as well. Collecting one Orcus Blood from defeating Orcus and delivering it will complete the quest and unlock the Soldier Enma body armor in Ueno's shop.
Orcus later appears as a normal enemy in Camp Ichigaya. He can teach Flynn the Mamudoon, Energy Drain and Tetrakarn skills through his Demon Whisper.
"A death demon of Etruscan lore. A terrifying portrait of this demon was painted on the walls and tombs of the Etruscans. He was later given the same status as the Roman Pluto and Dis Pater, rulers of the underworld."
—Shin Megami Tensei IV Apocalypse compendium
Orcus can be found in Minami Sunamachi. He can teach Nanashi the Mamudoon, War Cry and Tetrakarn skills through his Demon Whisper. Orcus benefits from learning Darkness, support and Force skills.
"The god whose name was said to refer to the very underworld itself in Roman mythology. His huge and intimidating stature is often depicted in ancient Roman murals. However, he later was often associated with other Greek gods as time went on."
"I will not stop until I have sipped on a chalice of your blood!"
—Orcus before engaging the player
Since Orcus is the reborn form of Horkos, their battles are also similar. Orcus uses the same attack pattern and has the same weaknesses, but has more resistances. Orcus retains Horkos' unique moveset but some moves now have upgrades. Chomp is now changed to Taking a Bite but still retains its properties of being a Life Drain-esque attack. Grocery Run still summons a single Katakirauwa although now each Katakirauwa summoned is at level 46.