Blizzard has announced that this July it will release three missions that act as a prologue to Starcraft 2: Legacy of the Void. Whispers of Oblivion is a three-mission arc that tells the story of Zeratul, we've only met through Heroes of the Storm, Blizzard's moba. who we first met in the original Starcraft.
These missions will be available for everyone whether they own any Starcraft games or not.
Thanks to everyone who pointed out my mistake.