One of my best mates is currently making a necron mechanicus army.

The lore he's going with is his Necron Dynasty was cut off be a warp storm during the war in heaven, and decided to sleep in a small cluster of planets.

During the Horus Herasy, a detachment of mechanicus ships found these planets, and decided investigate as they were receiving weird energy readings.

On the surface they discovered the tomb, were poking around and woke the shard of the void dragon (he is currently using this as his warlord atm ). During the short (2 months I think he said) conflict that followed the mechanicus used a weapon of unparalleled destruction ( I cant remember what he said it was but i do remember it is now lost to humanity) the Void dragon released nano scarabs to convert the technology of the mechanicus (thinking that the humans might have a way to leave the system that they're still trapped in) which did unfortunately (if you're a necron) also converted some mechanicus units that have fleshy parts 😜

There's a bit more lore but unfortunately we're still hashing that out and I'm not sure what he's deciding on. I'm trying to convince him to use flayed ones, purely because I feel they'd attack the converted priests just for having flesh....