Flayed 1's.

During the end of the War in Heaven, the Silent King did not know you shouldn't destroy a C'tan. So he did. He killed a C'tan named Llandu'gor the Flayer. Luckily Llandu'gor wasn't that important to the fabric of reality. However his death did create a computer virus called the Flayer virus. This virus made Necron's want to wear skin and eat flesh even though they can't eat. Szarekh decided to break the other C'tan into shards rather then kill them.

The Flayed 1's wait in a pocket dimension for other Necron's to fight enemies with flesh. They then help the Necron's in the fight. However the other Necron's will not be greatful for the reinforcements as touch can cause the virus to spread. So if the Flayed 1's don't leave when asked, the uninfected will wipe them out.

There are Necron's that are immune to the virus like Valgûl, but they are few and far between.