We'll Be Back
The Necrons have a remarkable ablility to repair themselves due to their use of the Necrodermis Living metal. Living metal has an uncanny ability to 'flow' back together, closing bullet holes, mending gashes and tears, or even reattaching severed pieces with little delay. The background basis for this lies primarily in the construction of living metal itself which is a mysterious and inscrutable substance, as well as the additional repair mechanisms provided by Tomb Spyders and Scarabs for maintenance purposes. Even the most powerful armaments have no way to stop this terrifying resurrection mechanism so long as an advanced repair source is nearby (such as a Resurrection Orb). Necrons too damaged to repair in the field disappear with a burst of eerie green light, only to manifest for repair in one of the Necron Tomb Worlds to be brought forth again.
The name for this ability is a possible reference to the Terminator franchise: the titular robots are a race of machines that wish to exterminate humanity and "I'll be back" is a catchphrase of the most distinctive Terminator units in the films (the T-800 Model 101. In addition, another Terminator unit, the T-1000, is made out of a fluid metallic substance that can regenerate after sustaining damage, much like a Necron's necrodermis.