In this panel, Lucifer refers to something named the Dark, that challenged Heaven and caused a civil war in Hell.
I understand that the Dark is the cosmic entity that was there before God, etc. (I have read the wiki page as well); I am interested in exact events that happened in Hell and forced Lucifer to form a triumvirate.
I have read the relevant Swamp Thing arc, but it did not describe what passed in Hell at the moment. I've also read The Sandman and Lucifer, which don't touch this matter.
If I remember this correctly, Lucifer alone is capable of wiping out the entire host of demons - what could possibly challenge him? And what possibly force his pride to accept this?