The Ultralisk is a late-game Hive-tech unit that is only available once the Zerg player has an Ultralisk Cavern. It is very expensive costing 200 Vespene Gas, but has a great deal of health and its speed is still on par with Zerglings. It features two specific upgrades, for movement speed and unit armor, both at the Ultralisk Cavern. Its attack and armor can be further upgraded at the Evolution Chamber. Because of its speed and the hit points sustained, Ultralisks are quickly brought into battle and can frequently survive substantial fire.
It also has a unique upgrade called Chitinous Plating which adds +2 armor. It is the only upgrade in the game to add armor to a specific unit. This upgrade, when combined with the Zerg's normal armor upgrades, makes the Ultralisk an extremely tough unit. Ultralisks are able to absorb a great deal of damage, particularly in drawing fire while units paired with it, such as Zerglings, can deal substantial damage. While the Ultralisk's attack is unspectacular (about that of a pair of Zerglings) given the significant cost, it has much greater lasting power and is fearsome when massed. Since they only take one Larva to produce and consume four supply, they are perhaps the most production-efficient unit in the game.
The Ultralisk is produced at any Hatchery, Lair, or Hive after the required Ultralisk Cavern has been morphed. The unit does not have abilities such as casting spells. Ultralisks only attack ground targets.
Ultralisks are common in the ZvT and ZvP match-ups, but are rarely seen in ZvZ due to it focusing on low economy and air units.
The fact that it is a large unit and so expensive gives it different properties in different match-ups, but generally speaking Ultralisks give Zerg a larger effective health-pool on par with that of other races' units. In the Ultralisk/Zergling ZvP strategy, the Ultralisk is paired with Zerglings to absorb hits from the Protoss units and block off their pathing while the Zerglings deal a large amount of damage throughout the base where the Ultralisks are not preoccupied.
Ultralisks are often used in the late game vs. Terran. This is particularly common in the case of macro ZvT strategies where the Zerg will attempt to out-expand Terran and win with mass production and speed of movement.
In such strategies, Ultralisks are usually built after the Zerg player has four Vespene Gas Geysers being harvested and at least +1/+1 Melee Attack/Carapace Upgrades. In combination with Defilers and Zerglings, Ultralisks can both absorb a lot of damage and kill Marines fairly quickly. Fully upgraded Ultralisks are one of the hardest units to stop for Terran Bionic play. In order to maximize the effectiveness of Ultralisks, the Zerg player will often destroy the opponent's Science Vessels with Scourge so that the Ultralisks improve their survivability in the face of mass Irradiate. If the Ultralisks are Irradiated, however, they can be directed into clumps of enemy Marines, which must also endure (and die to) the Irradiation spell. Against Mech Terran, Ultralisks are less viable due to the Siege Tank's very high damage per shot. As a result of the potency of this attack, even the Ultralisk's high armor is rendered, by and large, negligible. As a fire-drawing unit, however, Ultralisks are still effective in supporting improving the survivabilities of large numbers of Zerglings, Hydralisks, and Mutalisks in the late game.
It is important to note that although the Ultralisk is a powerful unit, it needs to be used in conjunction with the Defiler to reach peak effectiveness. When Plague is cast on the enemy troops, the Terran cannot Irradiate any Ultralisks because the splash damage will kill any Marines the Ultralisk approaches. In addition to this, Dark Swarm will help immensely when engaging M&M with Ultralisks.
Ultralisks also are very effective against any Firebats that the Terran player might mix into his army. Fully upgraded Firebats do a maximum of four damage to Ultralisks while Marines only do three damage to Ultralisks.
Commonly used in the late-game, the Ultralisk is most often seen combined with attack speed upgraded Zerglings (Cracklings) and Dark Swarm. While the Ultralisk deals a large amount of damage by itself, its primary purpose is to absorb most of the damage, while allowing the Cracklings (the real damage-dealers in this deadly combination) to dish out massive amounts of damage.
The Ultralisk will often draw powerful spells like Psi Storm, thus depleting available High Templars of their Energy, as follow-up units deal damage on the Protoss troops. Ultralisks are particularly powerful against static defensive structures like the Photon Cannon because of their heavy armor and potent attack.
Typical Zerg mirror match-up games end relatively quickly before Hive tech is reached. If it is slow enough up to that point however, Ultralisks all but hard-counter Zerglings, but are themselves hard-countered by groups of Mutalisks due to their inability to attack air targets. An Ultralisk/Zergling/Hydralisk attack depends on having the damage to finish the game before Mutalisks can respond or be made to respond.