Blizzard revealed that their expansion to Starcraft 2, Legacy of the Void, will come with three new prologue missions called Whispers of Oblivion. The prologue missions will serve to connect the campaigns of Heart of the Swarm and Legacy of the Void. These missions will be made available to all players for free, but players who pre-ordered Legacy of the Void will get the first chance to play them during the beta of the campaign.
Whispers of Oblivion follows Zeratul, a Protoss ally of Kerrigan who appeared in the original Starcraft and Starcraft 2, in the final portion of the Xel'naga prophecy. The new missions complete the stories of Kerrigan and Raynor. The new missions ease players into playing with Protoss troops, who will be the main forces in Legacy of the Void. The first mission, Dark Whispers, will require players to build quickly to take out the Zerg in a rescue mission. The second, Ghosts in the Fog, forces users to work with highly limited and sporatic resources. The third mission is called Evil Awakens.
While the official date has not been announced, we know that the beta of these missions will become available sometime in July 2015.
Note: Some information was inaccurate when first written during E3. The article has since been changed to reflect the most accurate, up to date information.