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.
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.