Stratholme used to be one enormous dungeon with two sides, and players could choose to do one, or both of the sides, without leaving the instance. Now, however, the two sides have been divided into separate instances, and you can no longer reach one side from the other. The first side, which used to be nicknamed "Live Strat", used to deal with the Scarlet Crusade's stronghold in the damned city, and your group unmasking the treachery within the organization.
In the former version of the instance, you fought through the streets of Stratholme, still burning from Arthas' rampage, and arrived at the Scarlet Crusade stronghold, which was under attack from Scourge forces.
Your group fought its way through the hallways of the stronghold, until you arrived at the throne room to confront the leader of the Scarlet Crusade, Saidan Dathrohan.
After a brief fight, Dathrohan can keep his secret no longer, and transforms into Balnazzar, a Pit Lord of the Burning Legion, the Demonic group that first introduced the Scourge to Azeroth. Balnazzar reveals that from the beginning, the Scarlet Crusade had been secretly led by the creators of the very threat they were trying to defeat.In the current version of the instance, Balnazzar has foregone the charade, and appears as the Pit Lord that he really is. He has reanimated the corpses of the fallen Scarlet Crusade forces, which explains why all of their remnants in the Plaguelands are undead.
Meanwhile, in the Undead Side, you begin in an open plaza of the burning city, which is swarming with Scourge. You need to make your way around the streets and destroy three mini-bosses and the ziggurats that they guard that are powering the defenses of the slaughterhouse, where the final boss is waiting.
Once you have access to the Slaughterhouse Square, you need to deal with one of the Baron's abominations before getting inside.
The final boss is Baron Rivendare, who is one of the Lich King's chosen Death Knights. The current in-game version has changed, as the original can now be found in the first raid zone of the Wrath of the Lich King expansion, but the fight has remained the same, and there is still a small chance that the boss will drop the reins to his skeletal horse, which players can use to ride one of the game's most coveted mounts.