Lucifer is one aspect of Satan, the King of Hell and the ruler of evil. His first appearance in the Multiverse is in the Hell's Broken Angel storyarc, where Deathwalker, a soul damned in a low corner of Hell, enters a pact with him. The terms of their pact state that as long as Deathwalker can find, kill, and deliver pure, innocent souls to Hell, then he will be spared from eternal torment for a time.
Lucifer was once among the most glorious of Seraphim in The Heavenly Realm, before he turned against The Divine Trinity, believing himself to be more worthy to rule the Multiverse. Therefore, he rebelled against the Trinity, accumulating an army of Cherubim and Seraphim. The resulting war nearly ended the still young Multiverse.
However, he ultimtely lost the battle and was cast down into Hell.
(More will be revealed as time goes on- this will have many connections to the modern Multiverse)