Flayed Ones are twisted and ghoulish Necron terrors afflicted by an ancient infection that breeds a hunger for organic flesh in them. ... They appear from an unknown pocket dimension of their hideous kind to join the Necron armies in battle, though never by invitation from the Necrons themselves.

Are flayed ones any good?

While the Flayed Ones are really good, they also suffer from some big downsides. The main thing is that while they have a solid leadership, they can get run down if you lose too many of them in one round of combat. They are not fearless and getting fearless on them is quite difficult.

Why do Necrons wear skin?

Flayed Ones are Necrons who have succumbed to the Flayer Virus of the C'tan Llandu'gor, a process which drives them insane with hunger for blood and gore. They wear the skin and body parts of their opponents, spreading mass terror across enemy lines.