The vampire lord’s history is well documented elsewhere and will not be repeated here; rather, the events leading to this august personage’s membership in the Necromancers of Skahlehn are of more immediate value.
Strahd‘s domain of Barovia was one of the “Fading Lands” to overlap some of those on Oerth; Barovia is now a tiny estate found in the Lands of Darmen in the former Great Kingdom. He was discovered by the drow witch Greyanna, who immediately reported back to the High Potentate, Baalshezidek. The lich sent greetings to the vampire lord and suggested an alliance. Seeking allies to better inform him of the goings-on in this new land, Strahd agreed. Baalshezidek was pleased since he hoped to learn something of the unique energies of Ravenloft through Strahd.
As a measure of his goodwill, Baalshezidek manipulated the Scourge of the Underdark to enter Strahd‘s castle in the company of Tamarina, the girl who was the reincarnation of his lost love, Tatyanna. Unfortunately for the vampire, it was revealed how the hero Tempus was also a reincarnation: that of Strahd‘s hated brother, Sergei. After many struggles, the heroes defeated Strahd, seemingly for good. However, it is unknown whether the unique abilities conferred upon Strahd as being a Lord of Domain carried over with him on Oerth; if they did, he is unlikely to be permanently destroyed.