This is according to a recent post by Blizzard on its Battle.net website where the missions have been opened up to everyone. Prior to this, the missions were only available in the Legacy of the Void beta. To access the prologue missions, just fire up StarCraft 2, head on over to Campaign and select Legacy of the Void > Prologue.
According to Blizzard, “These missions focus on Zeratul and serve to bridge the story between Heart of the Swarm and Legacy of the Void.” As for Legacy of the Void, the game will be released on the 10th of November next month. If you’re in it purely for the story, then Whispers of Oblivion will definitely be worth checking out.
Legacy of the Void is also expected to conclude the ongoing StarCraft 2 story, meaning that future expansions of the game will start something new entirely. What the future holds for the StarCraft franchise remains unknown, so for now fans should probably soak up as much content as possible.