Seven Mortal Sins

Belial seeks out Satan and tells her that Lucifer has managed to subjugate Beelzebub (conveniently neglecting to mention that Beelzebub did so willingly), inciting Satan to seek out the fallen angel and punish her for this supposed transgression. Leviathan engages her in battle to defend Lucifer, but with the city currently gripped by wrath and thus fueling Satan's power, she is easily defeated. Encouraged by Beelzebub and her fondness of Maria, who is now imprisoned in the palace of the Seven Sins, Lucifer faces Satan and her fleet of conscripted Navy warships. When she is pushed to the verge of defeat, Beelzebub brings Leviathan back from unconsciousness, and the tide of battle is quickly turned. After relocating to the surface on the Moon, Satan confidentially unseals her portion of Lucifer's Garb of Punishment, returning a portion of her powers, and faces off against her opponent in single combat. Despite the Demon Lord's vast level in power, Lucifer manages to beat Satan, who admits defeat and willingly agrees to open the gates of Hell so that Lucifer can carry the final fight to Belial herself. (Source: Wikipedia)

For her failure to subdue Lucifer, Belial imprisons and torments Satan, forcing her to guard the gates to Hell against Lucifer's impending intrusion. But prior to her invasion, Lucifer has utilized her new abilities as a demon lord to draw power from the lingering pride in every human's soul, thus gaining entry. As Lucifer and Leviathan make their way down to the lower levels of Hell, Belial attempts to rally the other Mortal Sins, but except for Astaroth they all refuse to follow her any longer. After struggling with a lewd Charon, Lucifer and Leviathan reach the Palace of Minos, where they are met by the other Seven Sins. It is then that Lucifer reveals why she was cast from Heaven: God had issued an order to the Seven Lively Virtues to purge both Hell and Earth, even though the corruption of humanity - and thus the existence of Hell - is part of an inevitable course which cannot be broken even by God's power; and Lucifer was banished when she protested against the perpetual repetition of this measure throughout the millennia. It is also revealed that Belial knew about this, and has struck a deal with Michael to have Lucifer delivered to her for eternal damnation in exchange for the release of all souls trapped in Hell and getting exempted from the purge. Astaroth, refusing to believe that Belial would betray them all, attempts to attack but is held off by Leviathan, while Lucifer continues her journey down to Cocytus. (Source: Wikipedia)