Strahd von Zarovich is the lord of Barovia and the master of Castle Ravenloft. He is an immortal vampire, desiring endless power and control. His undead curse is linked to the curse of the land of Barovia that prevents anyone from leaving.
Before he was a vampire, Strahd was simply the lord of the castle Ravenloft. He had a brother, Sergei von Zarovich, who was in love with a woman named Patrina. Strahd was in love with Patrina as well, and wanted her for himself. In anger, Strahd poisoned Sergei and Patrina at their wedding and cast the city of Carcosa in which they were married into another plane, creating Dim Carcosa. Since then, Strahd has been attempting every 100 years to open the Seven Gates to restore Carcosa to the world and claim his bride.
Strahd's goal during the Dragon Friends' time in Barovia is to cast a spell that will summon an undead army from another plane. As a component for this spell, he needs to sacrifice the thing he loves most in the world. His trouble is that he has not loved anything since Patrina. He then sees Ireena Kolyana who looks exactly like Patrina and decides that she is Patrina reincarnated. He kidnaps her and keeps her at his castles, where the Dragon Friends go to rescue her.
He can only be defeated by the Name Sword of Sergei von Zarovich. His death by the sword lacks drama, however, because as he is killed he is having uncontrollable diarrhea because of Freezo's Cursed Coins.
Strahd is brought back to life by Grimm Saltback and the Order of the Lampwick in Undermining a Dracula (5.16), believing that returning from his own death may be what was required to open the sixth gate, the Returning, which he had previously tried and failed to open by simply walking back into town. Although a portal to Carcosa was opened, it did not persist or expand; but the living Strahd escaped as a bat swarm.