The Countess in Hell has a chance of dropping the Key of Terror, just as the Summoner in Hell may drop the Key of Hatred and Nihlathak in Hell may drop the Key of Destruction. These three keys, when combined in the Horadric Cube in Hell Harrogath, creates, randomly, one of three portals, to the Matron's Den, the Forgotten Sands, or the Furnace of Pain. Each will only open once in a given game, so having three "Key Sets" guarantees one of each.
In the Matron's Den lies Lilith, who drops Diablo's Horn.
In the Forgotten Sands lurks Uber Duriel, who drops Mephisto's Brain.
In the Furnace of Pain dwells Uber Izual, who drops Baal's Eye.
These three organs, when combined, lead to Uber Tristram, where Uber Mephisto, Uber Baal, and Pandemonium Diablo await. Their destruction earns everyone the Standard of Heroes, and a single Hellfire Torch.
This is the so-called Pandemonium event, and was the primary addition to the game in the v1.11 patch.