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.
Immortals 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 and are even able to shrug off rounds fired by siege tanks. When deployed, immortals give critical support by eliminating enemy artillery and ranged attackers. However, the specially hardened energy shields of immortals only activate when struck by powerful attacks, which leaves them more vulnerable to the pinprick attacks of lesser foes.
Ultimately, immortals are a dying breed. Soon, none will remain.
The immortal is geared against small groups of armored targets. The hardened shield allows the immortal to survive even when outnumbered by units with slow but powerful attacks; conversely faster firing units, typically dealing low damage per attack, work well against immortals. In the campaign, immortals are good defenders against Maar and other hybrids.
Immortals dropped by warp prisms may conduct raids against enemy structures and expansions.
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 hardened shields reduce incoming damage to 10. After the shields are depleted, the immortal takes damage directly to its hit points.
Damage calculation is as follows: when the immortal is attacked and it has shields left, the damage is calculated in the same way as any other unit (including shield upgrades). If the calculated damage is 10 or below, it's applied directly (exactly the same as any other unit). If it's higher than 10, it's reduced to 10 by the Hardened Shield mechanic, then applied.
A battlecruiser's Yamato Cannon attack will not activate Hardened Shields and other spell attacks ignore the hardened shields.Nuclear missiles ignore the hardened shields ability. The ghost's EMP ability can be used to remove an immortal's shields.
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 you open the immortal portrait on the map editor's visualizer.
↑ 1.01.11.2Blizzard Entertainment (2007-05-19). Immortal, Blizzard Entertainment, Retrieved 19 May 2007
↑Hardened Shields reduce the damage of incoming high damage attacks to 10 if Immortal's shields are up. After Shields are depleted, the damage hurts the Immortal's life directly.
About the Shield upgrades, it goes like Damage - Shield Upgrades and then - Immortal's Hardened Shields. For example, if your Immortal shoots the enemy Immortal with fully upgraded shields, the damage will be 10: 35 (20+15 vs Armored) - 3 (full shield upgrades) = 32, and it's more than 10, so the enemy's Immortal takes only 10 damage by Hardened Shield. But if your Disruptor shoots the enemy Immortal with fully upgraded shields, the damage will be 7: 10 - 3 (full shield upgrades) =7 (it's below 10 damage and so Hardened Shield won't do anything).
It's little bit complicated to explain how the damage goes but basically it is like, Damage - Shield upgrades - Hardened Shield. Cydra. 2009-05-28. Karune, question about Hardened Shields. Battle.net StarCraft II General Discussion Forum. Accessed 2009-05-28.
↑A good amount of zerg units are affected by the immortal's Hardened Shell. They are particularly effective against roaches, ultralisks, and spine crawlers. Also, Psionic Storm and Nuclear Strike do not appear to be affected by the Hardened Shell as the immortal I just nuked disintegrated gloriously. Avarius. 2009-12-14. Immortal's Shield and Zerg Units. Battle.net StarCraft II Gameplay Discussion Forum. Accessedc 2009-12-15.
↑Ghost does not have EMP ability any more at the current development stage.
For your information above questions,
1. Immortal's hardened shield is passive and there is no additional upgrade to get it.
2. Ghost's EMP took out Immortal's shield.
3. Once Immortal's shield was taken out by EMP, Immortal took full damage from Siege Tanks, which means there's no more hardened shileld. Cydra. 2008-11-26. Dear Blizzard: EMP + Tanks vs Immortals. Battle.net StarCraft II General Discussion Forum. Accessed 2008-11-26.