Well from what I know, the Necrontyr were originally based on a single world, then when they got hold of a C'tan they started to create an empire.
I think they then started to encounter Old Ones and in the end the C'tan known as The Deciver fooled them into being en-tombed in their current metal bodies. By that time im sure the Necrontyr whould be spread across may systems, and hundreds of worlds. That would mean lots of Necrontyr.
Now you say they were a "dying race"? Where did you get that from? They most likely all changed to metal bodies eventually, or the last necrontyr were hunted down and killed by the superior Necrons becuse they did not follow the C'tan.
The way they got changed is basically most of their mind was transfered to the Necron bodies. The sneeky bit about it was that their freewill and anything else that would cause them not to follow the c'tan.
The Necron bodies are made out of 'Living Metal'. Basically a metal that can repair itself to its orignial form. |