If a Necron army is reduced to 25% or less of its original number of models (in other words, 75% of its models, rounding fractions up, are destroyed), it will disappear in an eerie fashion, leaving behind nothing of its presence. This gives an automatic victory to the enemy, regardless of the victory conditions of the scenario being played. For example, if an army of 50 Necron models is reduced to 12 models or less, it will disappear from the battlefield. Remember that you only count models with the Necron special ability, so C'tan, Pariahs, Scarab swarms, Monoliths and Tomb Spyders do not contribute to the total number of Necrons in the army or to the current number of casualties. However when Phase Out occurs, the whole Necron army, including models without the Necron ability, phases out.
Phase Out is calculated at the beginning of the Necron turn after all We'll Be Back! rolls have been taken.
Errata — Chapter Approved: Codex: Necrons Q&A (v2.0): Because the check happens after 'We'll be back' rolls, models that self-repair that turn count towards staying above the threshold — for example, an army reduced to 15 of an original 60 models (below the 25%/15-model threshold) that then recovers 2 models via 'We'll be back' would be checked at 17 models, not 15, and would not phase out. The threshold is reached as soon as the on-table total drops to 25% or less of the original — for an army of 99 Warriors and a Lord (100 models), that means phase out is triggered once the army is reduced to 25 models or fewer, not specifically upon losing the 75th model (the same point in this example, but the trigger is the resulting model count, not the count of losses).