The following special rules apply only to those models with the Necron special rule specified in their profiles.
Necrons have a remarkable ability to self-repair even the most horrendous damage. Any Necron model that is reduced to 0 Wounds, or would otherwise be removed as a casualty, remains on the tabletop and is laid on its side to show that it's damaged. Damaged Necrons ignore the normal coherency rules and cannot be attacked in any way – they are seen as just more battlefield debris.
At the start of every Necron turn, damaged Necrons may self-repair. Roll a D6 for every Necron capable of self-repair. On a roll of a 1-3, remove the model as normal. On a roll of a 4-6, the Necron is repaired and stands back up with 1 Wound remaining.
A Necron cannot self-repair if it was destroyed by a close combat weapon that allows no Armour save or any weapon whose Strength is twice the Toughness of the Necron concerned. This can be over-ridden by the Resurrection Orb (see the Necron Armoury). Additionally, the self-repair ability only works if the wounded Necron is within 6" of another model of the same type, although not necessarily of the same unit.
The repaired Necron will immediately be placed in coherency with the closest unit of the same type. Once joined with a unit, the Necron moves and fights with it for the rest of the game. If the nearest unit is in close combat then the repaired Necron may be placed in combat with any of its opponents as long as it maintains coherency. It may not contact a new enemy unit and does not count as charging.
Necron Lords
Necron Lords do not have to have another model within 6" in order to self-repair. If they self-repair, they will stand up with 1 Wound, not their full 3. If the Lord's last Wound is lost to a close combat weapon that allows no Armour save or any weapon whose Strength is twice the Toughness of the Necron Lord then it cannot self-repair unless it or another Necron Lord within 6" has a Resurrection Orb. If several attacks hit simultaneously, assume that hits that do not allow self-repair strike ahead of hits that do.
Tomb Spyders
If a Necron cannot self-repair because no model of the same type is within 6", it may still self-repair if there is both a Tomb Spyder within 12" and another model of the same type on the battlefield. If the Necron recovers then it will immediately join that model's unit as normal.
Victory Points
For Victory point purposes, base all calculations on each unit's original and final strengths only. So if a unit of Warriors started with 16 members, lost 10 casualties but gained five Warriors from other units, they would finish with 11 Warriors and would not count as being below half strength. Similarly, if the same unit lost nine casualties then it would be worth half its points to the opponent, even if those casualties have self-repaired and joined other units.
Errata — Chapter Approved: Codex: Necrons Q&A (v2.0): Necrons that are Tank Shocked, choose to stand & shoot, miss, and are then run down by the vehicle may not attempt a 'We'll be back' roll — the weight of the vehicle is enough to crush even a Necron beyond self-repair. A damaged Necron lying on its side does not count as "a model of the same type" for another Necron's self-repair check — only active, standing models count, so casualties can't chain off each other's presence. A Necron that has failed its 'We'll be back' roll, or was ineligible to attempt one, is removed at the start of the Necron turn unless you intend to use a Monolith portal to teleport the unit during that same Movement phase, in which case leave it on its side until the end of the Movement phase; if you change your mind about using the portal, remove it immediately. A damaged Necron still lying on its side when the game ends counts as a casualty even though it never got the chance to attempt a roll. If a unit that has fallen back is rejoined by self-repaired models from other units, the combined unit remains broken and continues to fall back rather than rallying, but if the reinforcements raise the unit's current strength above 50% of its original strength, it becomes eligible to attempt to regroup; any subsequent Morale check the unit takes is based on its current strength, not its original strength. Being destroyed by an effect that nullifies Armour saves without literally being a close combat weapon — such as the Act of Faith Divine Guidance, a Tyranid's Rending Claws, or a Chaos Daemonic Talons attack — still counts as being destroyed by a weapon allowing no Armour save for 'We'll be back' purposes, so no roll can be attempted unless a Resurrection Orb is in range.