I just killed the boss of the den and not a single thing happened.

Players of Hell Difficulty Realm games are hereby warned once again,
that a series of new and challenging tests await you! The answer lies
within Diablo's Bosses, which span across the world from the Den of Evil
to the Throne of Destruction...

wtf? I get blue items. I guess the secret is if you wear all blue items you can go fight lilith.