The StarCraft II website announces Whispers of Oblivion, a three mission prologue to bridge the gap between the previous Heart of the Swarm and upcoming Legacy of the Void expansions. At the E3 PC dealie last night they announced this will be free for everyone. Here's a trailer and here's word on what this will offer:

What is Whispers of Oblivion?
Whispers of Oblivion is a special three-mission prologue series that has been created to bridge the storyline between Heart of the Swarm and Legacy of the Void.

What can we expect from these missions?
In Whispers of Oblivion, players will join Zeratul for the first time since Wings of Liberty to unravel the final piece of the Xel’naga prophecy.

How can I play these new campaign missions?
We will be making Whispers of Oblivion available to all players before the game’s release, but prior to that, we will be providing early access to the Prologue missions with pre-purchase of the game. Look for more details sometime in July.