While StarCraft II is known for its multiplayer component, some of us are mostly interested in the campaign… and Arcade mods, but there's no news on that front. Legacy of the Void is the end of the StarCraft II trilogy, which is said to finally deal with the hybrids that were introduced in the secret missions of Brood War and StarCraft II: Wings of Liberty. They played a larger role in Heart of the Swarm's campaign although that did not even have unlockable missions, so they wouldn't exist otherwise.
StarCraft II: Legacy of the Void does not yet have a release date, but there will be a mini-campaign released for free before it launches. StarCraft II: Whispers of Oblivion (or is that StarCraft II: Legacy of the Void: Whispers of Oblivion?) are three single-player missions that will be released in July. Those who pre-purchase Legacy of the Void will get the missions first, which might mean that everyone else needs to wait until after July to play them… or not. That said, if you are patient, you do not even need to own StarCraft II at all. Free to all, but timed-exclusive for those who pre-order.