From Lexicanum

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The Necrons have a remarkable ablility to repair themselves; in the game, this is reflected in a Special Rule known as "We'll Be Back!". This gives the Necrons incredible staying power against all but the most powerful weapons, making them an enemy dreaded by all races in the Warhammer 40,000 universe. It is often said amongst commanders of more orthodox armies that the Necrons are too powerful a force on the battlefield. Necron Players would of course point out that their squads lack heavy weapons and that the army rules were play-tested by Games Workshop like all the other armies whilst in the making.