Mundus the Dark Emperor (lit. "World" in Latin) is the primary antagonist of the first Devil May Cry, and his defeat at the hands of Sparda two millennia ago sets the scene for the series.
Two millennia ago, he was born into the depths of the Demon World, and quickly rose to power. He planned to lead an assault on the Human World. However, one of his chief generals, Sparda, "awoke to justice". Sparda then single-handedly defeated both Mundus and his armies and sealed the main gate into the Demon World.
At the end of Devil May Cry 3, as Sparda's son Vergil wanders the Demon World, he comes upon Mundus and challenges him. However, he is defeated, and transformed into Nelo Angelo.
In Devil May Cry, Mundus attempts to escape into the Human World by opening a gate on Mallet Island. At the same time, he uses Trish to manipulate Sparda's other son, Dante, into coming to the island so that he can be killed. Although Dante does come to the island, he is able to defeat Mundus and his minions. Mundus then falls back into the Demon World after Dante blasts him with super lightning charged Ebony and Ivory. To use the super charged Ebony and Ivory, you must have at least three Devil Trigger Gauge symbols full.
In Devil May Cry 3, and during some parts of Devil May Cry, Mundus appears as three glowing orbs arcing with energy. It is uncertain if this is his true form, or merely an avatar. Mundus also appears as a marble statue of a three-eyed, angelic youth with long hair. This form then takes on an appearance of a older, bearded man with a big hole in his chest (Probably a scar from his battle against Sparda). During his final battle with Dante, the marble breaks away, revealing a grotesque, writhing being made of lava, with three eyeballs and a mouth attached by stalks to its head.
Mundus appears to have been worshiped by the residents of Mallet Island, as there are many statues to his angelic likeness situated about the castle, and Dante comes across many clues of strange rituals during the game. However, it is also possible that these were instead constructed by the demons Mundus fills the island with.
The mundus Cereris was a pit in Roman mythology. It contained an entrance to Hades, the underworld, which was ruled by Pluto, the Roman god of the underworld; Mundus is identified as Pluto twice within Devil May Cry. The stone covering the pit, known as the lapis manalis, would be removed three times a year, and when it was removed it was believed that the spirits of the blessed dead would commune with the living.
Mundus' three eyes are likely a reference to Satan, who is portrayed with three faces in Dante Alighieri's The Divine Comedy. Satan is also considered the "prince of this world" and the "prince of darkness". Mundus shares both of these titles.
