Immortals are the Necrons' shock troops, deployed where regular Necron Warriors are insufficient. They used to be the soldiers of the Necrontyr Empire, and generally consented to the bio-transference process -- after all, an immortal living-metal body is a pretty big step up from a short-lived cancer-riddled sack of meat. While the soldiers were not nobles, they did have knowledge and experience that was valuable in the coming war with the Old Ones, so they were given bodies with better armor and superior weapons. They even retain some of their personalities, and are capable of some conversation, though it is basically limited to matters of battle and their voices are completely devoid of emotion (which is very annoying for the more hammy Lords who try to rile them up with inspiring speeches). They're a bit like the Cybermen from Doctor Who in some ways (which may not be a coincidence, given how prominent Doctor Who is in British popular culture). In the 7th edition codex, it is shown that Immortals do remember much of themselves, but thanks to their programming only feel rage and pride; a short story shows an Immortal taking the time to slowly crush a human skull for damaging his leg rather than quickly killing the offender.
They are veritable giants and big-as-a-Terminator huge, towering over even a Space Marine. If anything, bioconversion has allowed them to prosecute war with an even-greater efficiency. Unlike the Necron Warrior, who will fall back on defensive protocols should it be without orders, an Immortal will strive for victory using every tactic and strategy at its disposal if left to its own devices. An Immortal is incapable of adapting to new modes of warfare, but war has, even with technological advancements, remained the same across the eons, and an Immortal is seldom baffled in this manner.
For all that lore, the only difference between Immortals and Warriors on the tabletop is that Immortals are slightly more expensive and get better weapons (gauss blasters or tesla carbines vs. gauss flayers) and a better armor save (3+ vs. 4+). In the old Codex, Immortals were Elites rather than Troops, and had better Toughness (as both had 3+ armor saves), but that changed with the new Codex.
|Forces of the Necrons|
|Command:||Cryptek - Destroyer Lord - Necron Lord - Necron Overlord - Phaeron|
|Troops:|| C'tan Shards - Deathmarks - Flayed Ones - Immortals |
Lychguards - Necron Warriors - Triarch Praetorians
|Constructs:|| Canoptek Spyder - Canoptek Wraith - Crypt Stalker |
Scarab - Tomb Sentinel - Tomb Stalker - Seraptek Heavy Construct
|Vehicles:|| Annihilation Barge - Catacomb Command Barge - Doomsday Ark |
Ghost Ark - Monolith - Sentry Pylon - Triarch Stalker - Tesseract Ark
|Flyers:|| Canoptek Acanthrite - Doom Scythe - Heavy Destroyers |
Necron Destroyers - Night Scythe - Night Shroud - Tomb Blades
| Abattoir - Æonic Orb - Doomsday Monolith - Megalith |
Necron Pylon - Obelisk - Tesseract Vault