Whispers of Oblivion is a free three-mission series that bridges the gap between Heart of the Swarm and Legacy of the Void.
It's been a couple year's since Heart of the Swarm's 2013 release date, and you'd be forgiven for not exactly remembering all of the plot stuff that has gone down since Wings of Liberty launched back in 2010. That's why Blizzard is aiming to refresh all of our Starcraft memory by giving current players a free set of prologue missions.
Titled Whispers of Oblivion, the three-mission pack was announced at E3's PC Gaming Show, and has been created to bridge the storyline between Heart of the Swarm and Legacy of the Void. Fans of the series may remember that back in Wings of Liberty there were a set of special Zeratul missions, and Whispers of Oblivion appears to be a direct continuation of that.
Other than that, all we know is that it will have some kick-ass Blizzard-quality cinematics. We don't know when it will be released, and whether it will be free for all players, or just those who bought Heart of the Swarm.