Zeratul

Summary

Zeratul is a Protoss Dark Templar Prelate who is best known for helping to reunite the Dark Templar with the Khalai Protoss that exiled them thousands of years ago. After Tassadar's death at the end of the first Protoss campaign, he becomes the primary character in the Protoss storyline. Zeratul is a very haunted character as it was his actions that led the Zerg Overmind to Aiur, which resulted in thousands of his kind being slaughtered by the Zerg. He was also forced to kill the leader of the Dark Templar, Matriarch Raszagal, after she was put under Zerg control by Sarah Kerrigan, the Queen of Blades. Zeratul plays an important role in Starcraft II: Wings of Liberty, showing Jim Raynor, the protagonist of the Wings of Liberty chapter, visions of his own memories (which are portrayed to the player in the form of Protoss missions) in order to send him an important message. It is as yet unknown whether Zeratul will survive the second chapter of the Starcraft II trilogy, Heart of the Swarm, but it is generally assumed that Zeratul will be the protagonist of the final chapter, Legacy of the Void. 
 
The message that Zeratul gives to Raynor is news of a vision seen by the Zerg Overmind, whose outcome depends entirely on whether Kerrigan lives or dies. If she dies then the vision Zeratul is given by the spirit of Tassadar about the obliteration of the Protoss and the end of the universe will come true. 
 

Voice Actor

Starcraft & Brood War - Jack Ritschel
Starcraft II Trilogy - Fred Tatasciore
  

Confusion over the original voice actor

Blizzard retained all the old voice actors from the first Starcraft but since Jack Ritschel died, they sent out a casting call for the role of Zeratul.  James Harper saw the casting call and remembered having played a role in SC1, though he didn't recall which character he voiced. At the time, IMDB had wrongly credited him with the role of Zeratul, so he contacted Blizzard looking to reprise his old role. However, Blizzard informed Harper that he actually played Arcturus Mengsk and that he would be reprising that particular role in SC2. The crew shared this story at Blizzcon '09.