Lilith

Lilith, The First Succubus, Seducer of Adam, Princess of Pride (following Lucifer's abdication). Mother of Lillin, friend of Lucifer, onetime flame of Leviathan.
Lilith never appeared in any episode but was first referenced in Season 2.
Season 2
Lilith was murdered by Asmodeus, triggering his coup of Hell. The act was recorded by the Voyeur, Asmodeus's silent, ever-vigilant servant.
Proserpine And The Sisters Of The Whip helped Fionn Corvin find the evidence and broadcast it to all of Hell.
Leviathan killed Asmodeus with his bare hands, in front of the gathered masses of Hell to avenge her death.