Prince of Darkness, studies the world with hungry eyes,
Prince of Darkness, ready to baptize you in lies.
--Alice Cooper, "Prince of Darkness"
Lucifer in the In Nomine universe, is the ruler of Hell and the former Archangel of Light. (It is not settled in canon whether his popular nicknames of "Prince of Lies" and "Prince of Darkness" represent actual Words held by him, or just colorful titles.) Of the celestials, only Yves and Michael are older than he is.
Lucifer was dismayed when God declared Earth off-limits to angels around 23,000 B.C. and the Archangel suggested an experiment to see whether humans were worth the trouble. God agreed and instructed Novalis to create the Garden of Eden, where two humans, Adam and Lilith would be placed in isolation from all other influences, to see if they would remain righteous and innocent when left alone. After Lucifer explained free will to Lilith, she walked out and had to be replaced by Eve.
The experiment, sadly, was contaminated when Lucifer sent one of his own Servitors into the Garden to "explain" things to the human couple. When they then sinned, Lucifer claimed it proved humans weren't so special, while God said it proved nothing and left the ban in place. The resentment fed a growing selfishness in the Archangel ... one he was quick to share.
Lucifer eventually gathered a third of the angels in Heaven and led them in a rebellion. Michael himself battled Lucifer on top of Gabriel's Mountain and threw him to the farthest reaches of the Symphony, ripping off the mountain's top and creating Gabriel's Volcano. After spending a few thousand years sealed in Hell, Lucifer and his followers got free with the help of Lilith and began going after humanity on Earth; God, in return, lifted the ban on celestial intervention in order to protect humanity from the newly-escaped demons, but ordered discretion. In order to best counter Heaven, Lucifer raised his most powerful followers as Demon Princes to counter specific Archangels, and set Hell to churning out new demons as fast as possible. The last major Prince raised was Nybbas, Demon Prince of the Media in the 1880s; the last minor one was Furfur in the late 1990s.
The Lower HellsEdit
While most Demon Princes and their followers dwell in the "highest" level of Hell (the one closest to Earth), Lucifer himself makes his home in the Lower Hells. Few go down there. The fewer that do and return report of the horror and the soul-chilling cold.
The first time Lucifer meets a celestial, he will entertain a request and maybe even fulfill it -- if it interests and intrigues him enough. (Hades help you if you waste his time or try his patience, though.) He is said to offer this boon to newly-met angels as well, though the sons of Heaven are probably wiser to refuse such a "gift."