Think of the StarCraft series, and you think of multiplayer. Its single-player campaigns are unavoidably overshadowed by the competitive scene, and news that Heart of the Swarm's reveal would be exclusively single-player leads to an irresistible question. Just how important is StarCraft's lore and single-player to StarCraft's fans? "Incredibly important," begins Dustin Browder, the lead designer of StarCraft II. "And I'll tell you what: I think most of the press ends up reporting on the multiplayer because that is what their fans are reading. But I know that most players just play the campaign. So there's a bit of a disconnect there between what we're talking about and what we're actually doing - a huge percentage of our players are coming just for the campaign." A huge percentage? "I don't know the exact percentages off the top of my head, but I do know it's well over 80 per cent that play significantly into the campaign." Browder later clarifies that figure, saying he doesn't know exactly how many of Wings of Liberty's players play single-player only, or when they dip into the multiplayer, but his point remains: the perception of StarCraft as multiplayer-only just isn't borne out by the stats. For a silent majority of players, the campaign is a Very Big Deal indeed. Heart of the Swarm, the second of what will eventually be three StarCraft II games, makes more sense in this context, because the switch to the Zerg as central characters has brought profound change to the campaign: this is now a hero-led RTS. Series staple Sarah Kerrigan, AKA the Queen of Blades AKA the genocidal Zerg leader, is the central focus and player-controlled character throughout HotS (and what a lovely initialism that is). It does seem a little weird that the Zerg uber-queen's weapon is a Terran canister rifle, but we're not going to tell her. This is a massive change. It would be easy to describe it in the context of hero-RTS archetype Defense of the Ancients or recent competitors like Heroes of Newerth, but the driving force here is internal. Wings of Liberty's campaign, as diverse and detailed as it was, did double service as single-player and as 'my first multiplayer trainer'. It was overshadowed by multiplayer, partially at least, because it was so similar in its mechanics. StarCraft's central character, and the focus of Wings of Liberty, is Jim Raynor - but he's just an ex-marshal with a battle cruiser and a can-do attitude. Kerrigan's scale is different. The entire Protoss race and Terran Dominion want her dead, and she happens to be the most powerful psionic creature in the universe, one who can control the entire Zerg species. As Raynor, you jump from planet to planet, pirating supplies and executing guerilla strikes. It's a merc's life. As Kerrigan, planets exist to be conquered, and death is a minor inconvenience. Kerrigan will be a constant throughout the missions with an individual tech tree: an on-field general that can tip battles and dictate engagements. "She's definitely the centre of most activity on the battlefield," says Browder. "It's fun to have that kind of power and now the lore supports it - whereas Raynor couldn't realistically stand up to 27 siege tanks, Kerrigan can. It matches the lore and at the same time lets us create a new gameplay experience so the player really feels they're getting something new, new types of challenges and things to accomplish." Before missions, Kerrigan's 'Battle Focus' (read: upgrade tree) can be set to either Specs Ops or Corruption, both of which grant active and passive abilities. The Corruption path is more instantly fun, built around a brilliant ability: Spawn Broodlings, which instantly kills a targeted non-massive enemy unit and spawns five Broodlings (tiny bundles of tooth and claw) from the corpse. But the choice between Spec Ops and Corruption isn't either/or. Kerrigan's Battle Focus can be changed between missions, and using Corruption upgraded our Kerrigan's Spec Ops abilities. Clearly the intention is for players to sway between the two, even if it's only to maximise one of the strands. The Spec Ops options were Pulse, a radius attack that dealt light damage and stunned all targets, and Psionic Shadow, which creates a doppelgänger Kerrigan with the same health and half the damage-dealing ability: this can be cast in such a way that you have three or four Kerrigans at once. They're nice enough, but don't really compare to bursting your enemies into little monsters. The build had two missions that lasted around a half-hour each. In the first, a Zerg Queen called Za'gara is attempting to establish her dominance over what remains of Kerrigan's swarm on Char (where Wings of Liberty's campaign climaxed), so it's hot Zerg-on-Zerg action. The setting is Char's acid marshes, a world of fetid green pools and rotting carcasses dotted with egg batches straight from Aliens. Browder tell us the two missions are "probably somewhere between the fourth and sixth" of Heart of the Swarm's campaign, but even this early and with the difficulty level locked on Normal it's clear Kerrigan's presence brings a new level of multitasking to single-player. After getting a base humming, the focus is on moving Kerrigan and a small force of Zerglings and Banelings around the map, collecting eggs and engaging groups of enemies as cost-effectively as possible, using Kerrigan's abilities to maximise your force's effectiveness. The 'transfuse' ability was missing from Queens in this build but, as an essential early game defence in multiplayer, it's certain to be unlocked in the final game. Za'gara herself can be engaged, and retreats once her health gets below a certain level, letting you effectively bully her out of the juiciest clusters if you time it right – while the increasing density of her forces is simply manna for the increasing number of Banelings you're producing. With a hundred eggs collected, the mission's climax sees around a hundred Banelings and Zerglings instantly spawned, which you can then simply move into Za'gara's base to destroy it with overwhelming force – a cathartic, explosive and definitive end to your game of cat-and-mouse. The second mission, 'Silence Their Cries', is set on the frozen moon of Kaldir and built around Roaches, a solid backbone of the Zerg army that deal good damage and are extremely hard to kill in large numbers. This begins with Kerrigan hunting another brood queen, Na'fash, but shortly after starting you find her corpse, along with many leaderless feral Zerg, and realise the Protoss have got here first. There's an interesting mechanic at work from the off here, one that feeds into a larger structural element of Heart of the Swarm: evolving. Neither Kerrigan nor her Zerg are adapted to the freezing temperatures and flash-freezes of the environment. But killing the yetis prowling around provides the genetic material to infuse the Zerg with resistance, which turns an obstacle into an advantage – because the boring old Protoss can't adapt as quickly, they're always incapacitated during freezes, allowing darting raids into otherwise fortified positions when the weather is right. Where the first mission was a war of attrition against another 'leader' character, this one has three fortified Protoss positions with clumps of Photon Cannons and Immortals that will crush any half-hearted attack. But using the weather to take out key pylons and wear down shields lets your Roaches establish themselves, and soon the Protoss forces are entrenched and being worn down by a combination of sucker punches and opportunistic strikes from Kerrigan. These engagements are a good guide to how Kerrigan will work throughout the campaign; put simply, your forces don't have the brute strength at this stage to attack such positions head-on. But by drawing fire with a Kerrigan doppelgänger, or simply bursting some defenders into Broodlings and instantly messing up the Protoss ranks, she creates opportunities for the Zerg to break through and swarm the defenders. She's an incredibly useful and hardy unit, simply in terms of damage dealt and health, but it's the tactical possibilities of her abilities that will let Kerrigan dominate Heart of the Swarm's battlefields. The mission ends with the Protoss getting ready to escape and spread the word of Kerrigan's return, setting up the next Kaldir mission as a brutal hunt to silence any survivors of Kerrigan's arrival. It shows how neatly Heart of the Swarm's narrative will slot into its new planet-based structure of mission 'sets'. In Wings of Liberty, you could hop from planet to planet; here, you'll commit to a planet's two to four missions and then see them through. Then there's the evolution of your Zerg forces. An NPC called Abathur, a rather slimy and physiologically queasy creation, watches over a between-mission evolution chamber where unlocked Zerg units can be upgraded and eventually changed into new types of monster. With the Roach, for example, 'mutagens' gathered in-game can be spent to upgrade it with Chitinous Plating (+1 armour), Bile Ducts (+2 damage) or Organic Carapace (+2 life regeneration) â€“ and when two of these are in place, the Roach can be evolved. Roaches can become Prowlers, which can move while burrowed (i.e. invisible to the enemy), or the irresistible Leeches. The latter gain health for every kill and heal rapidly when burrowed â€“ they're very, very difficult to kill. Lovely. Banelings are even better â€“ a suicidal melee unit with splash damage, the Splitterling evolution sees two 'medium' Banelings emerge scuttling from the first explosion and a further two small Banelings from each of their detonations. It looks like it should: an indefensible, swarming monster. Finally, Zerglings can evolve into Swarmlings, which hatch three to an egg (rather than two) and allow you to build a huge numerical force near-instantaneously, which is always the best bit about playing Zerg. Or you can plump for Raptors, which can leap to close with enemy units and have additional life. If the name isn't enough of a Jurassic Park reference, the in-game models are also altered with a fetching purple frill atop the creature's spine, just to make it clear how badass they are. If all this doesn't make it clear enough, let Browder lay it down for you: "This is about Kerrigan leading swarms of faceless creatures in to destroy the enemy." In-game she's an absolute beast and a significant change to the core mechanics of StarCraft – no other caster unit has approached this level of power. There are a few wrinkles in the build we play. Kerrigan's personality veers a little between determination to wipe out anything opposing her ("No-one gets out alive. No-one.") and more mawkish lines like, "The hunt is bloody. I can't believe I'm slaughtering innocents." But Browder is upfront about the dialogue and voice-acting being placeholder, while lead writer Brian Kindregan says they're still "smoothing out" the progression – and, of course, we played isolated missions rather than an individual arc. What they've clearly got right is the atmosphere. The hubs between missions, and the whole look of Heart of the Swarm's interface and environment, are infused with the moist and creeping personality of the Zerg, every surface chitinous, every hole infested. In the game, it's a sense of power, of inevitability – of striking hard and fast while building an unstoppable ball of claws and bile that soon rolls forth. It plays very differently to Wings of Liberty's campaign, and feels like it should: alien and insectoid. Heart of the Swarm is still evolution rather than revolution, of course, but it's a crucial step for StarCraft II as a whole. That's because, while it may be the modern king of the competitive RTS, its rank as a single-player experience is less clear. By letting you talk to the monsters, and control them as the biggest monster of all, Heart of the Swarm doesn't just promise change - it gives the silent majority their very own god of war. StarCraft II's multiplayer is an immovable object, and it will always be that way for the series. But Kerrigan, all sauced up and ready to rumble, feels awfully like an irresistible force. StarCraft II: Heart of the Swarm has not yet been dated, but is due to be released on PC.