Great Celestial War

The Great Celestial War is an event which, plot-wise, occurred long before MC arrives in Devildom. It is the name given to the war between God and Lucifer, where Lucifer's rebellion was quashed and all those who took part were banished.

Lilith was also killed in the war when Beelzebub failed to save her, choosing instead to save Belphegor. It is because of this that Beelzebub blames himself for the death of Lilith.