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. [Source: wikia]