The immortal is a protoss cybernetic war machine.
Immortals stem from dragoons. Their pilots live in a sterile nutrient solution saturated with microscopic machines, which helps preserve their mangled bodies. The machines repair aging tissue, destroy any foreign matter that might penetrate the cockpit (usually due to hull breaches) and regulate the pilot’s heart rate, brain activity, and biological functions. Immortals are some of the most advanced hardware ever devised by the protoss. Upon transplantation, immortals are concentrated in the name of Adun, as he was the one who best understood the meaning of self-sacrifice. 
Immortals stand over twice as tall as a terran wearing CMC armor. They are outfitted with two twin phase disruptors as their primary attack form; they give critical fire support to legions of zealots against ground targets, deployed when the protoss are faced with heavy enemy firepower. These blasts can easily punch large holes in heavy plating, but their limited field of fire prevents Immortals from engaging aerial targets. When deployed, immortals give critical support by eliminating enemy artillery and ranged attackers.
Immortals are equipped with armor weave that mitigates impacts through energy dissipation. Additionally, the hardened shields used by immortals are created by intertwining spectrums and wavelengths of varying intensity. Through this, these shields are incredibly dense, able to mitigate powerful impacts such as siege tank rounds or the blades of an ultralisk, through energy dissipation. When the remaining dragoons were turned into immortals, the protoss decided to outfit them with these shields in an attempt to further preserve their most ancient and venerable warriors. Originally, the shields only activated when struck by powerful attacks, which left immortals more vulnerable to the pinprick attacks of lesser foes. By 2505, immortal shields were heavily reinforced, making the immortal invulnerable to enemy attack for short periods of time. These shields were impractical for mass production.
Historically, crippled veteran Templar could volunteer to continue serving by being transplanted into dragoon exoskeletons. This honored tradition was broken by the fall of Aiur when the shrine dedicated to dragoon creation was infested by the zerg. Following the Brood War, crippled templar were transplanted into the new immortal exoskeletons instead. The first were in service by 2502. The remaining dragoons were transformed into immortals. As a result, immortals house some of the most skilled and venerable warriors of the protoss military.
Ultimately, immortals are a dying breed. Soon, none will remain. By 2506, only the most revered heroes of the Templar were allowed to become immortals, due to the resources required to produce them.
Immortals dropped by warp prisms may conduct raids against enemy structures and expansions.
Immortals are a good counter against roaches and ravagers. If a zerg player is using these units in excess, immortals can be considered a core unit.
While the immortal has shields remaining, incoming attacks that would deal more than 10 damage have the damage reduced to 10. Damage reduction from shield upgrades is applied after the hardened shields reduction. Attacks lower than 10 damage are unaffected.
Damage from spells, including nuclear missiles and EMP shots, is unaffected.
In the single-player campaign, the player may choose between the immortal, the annihilator, and the vanguard for their robotic walker unit. The vanguard has superior attack power and the annihilator has a heavy-damage shadow cannon ability, while the base immortal has its Barrier ability for superior durability and tanking abilities.
The hardened shield trait is removed, replaced by a barrier ability. In the campaign, the barrier initially absorbs only 100 damage, but lasts 10 seconds. Later, after "Brothers in Arms", barriers can absorb up to 200 damage.
Upgrades and Abilities
The immortal gains a barrier which absorbs 200 damage before falling. Lasts 2 seconds. Set to autocast by default; activates upon taking damage to health.
Artanis, Karax, and Fenix can build immortals in Co-op Missions. Artanis is able to upgrade his immortals from the start, while Karax unlocks immortal upgrades at Level 9. Fenix unlocks the ability for his immortals be controlled by Taldarin at Level 5, allowing one of them to pull targets together and the researchable ability to store damage within a target that explodes out to surrounding enemies upon death.
Upgrades and Abilities
The immortal gains a barrier which absorbs 100 damage before falling. Lasts 2 seconds, can be set to autocast.
The following section contains information from a previous version of StarCraft II which is no longer valid.
From a gameplay standpoint, immortals were conceived to act as "meat shields," as per its hardened shields. However, it has been found that players used them more aggressively than initially conceived.
The immortal's barrier ability was temporarily removed during development. It received a buff during the Legacy of the Void beta in order to encourage warp prism drops. The barrier originally gave the immortal 200 HP.
As an Easter egg, the immortal's portrait occasionally showed a "goldfish", although it appeared more often than it should. As a joke, Dustin Browder called the fish "Herbie". In the retail version the goldfish can be seen occasionally, but there's a long time between one and another appearance. It can be seen if the players opens the immortal portrait on the map editor's visualizer.