The Matron's Den is a special zone that can only be accessed during the Pandemonium Event. The zone can be entered by opening a portal in Harrogath; this requires the three keys of Hatred, Terror and Destruction. The Matron's Den is also where the demoness Lilith resides.
After her legions of Succubi and strongest ally Andariel were killed by mortals who successfully defended the Worldstone, Lilith, the Queen of the Succubi and matron of all demons, swore to personally intervene. She masterminded the revival of Diablo by manifesting herself in the mortal realm and performing a ritual of Dark Summoning. This ritual, long thought to be impossible by the most powerful mortal wizards, allowed Lilith to link with Diablo across realms, strengthening him until he was finally able to once again reincarnate himself in the mortal plane. Rage and a deep yearning for revenge boiled within the eyes of the Lord of Terror, and together he and Lilith resurrected the two remaining Prime Evils as well as Duriel and Izual, two of their strongest agents.
Diablo announced his grand plan - the Three were finally strong enough to spread Pandemonium throughout Sanctuary. Lilith could have easily conducted an assault on her own, but because she had been left severely weakened by the Dark Summoning ritual, she agreed to aid the Prime Evils and command some of their remaining forces. She retreated to her lair - the Matron's Den - where she guards the last remaining relic of the Lord of Terror himself: Diablo's Horn.