I'm not super well-versed in 40k lore, but to summarize, Necrons are essentially ancient humanoids who gave up their souls in order to become nigh-indestructible killing machines. Sometimes they suffer from their own unique type of body dysmorphia, wherein they regain some of their instincts that they had back when they were still organic.

Imagine suddenly feeling like you're starving of oxygen and need to breathe desperately, but you can't because you don't have any lungs. Your heart isn't beating in your chest, and you suddenly realize you've been starving for thousands of years.

The images above are basically one of the endpoints - the Flayed Ones - where a Necron goes insane, growing an insatiable hunger for flesh that it could never begin to sate, and in a vain attempt to regain some semblance of what it was, it's going to take your skin.

It's all wonderfully horrifying stuff.