Kaguro Necrons, are basically aliens who were flesh and blood at one point and lived on a planet with a toxic sun, so they were as a species short-lived and sick, and as a result, they were obsessed with discovering Immortality. and long story short they discovered a form of immortality ... they became robots made out of self-repairing metal (living metal) and are basically indestructible. They conquered about half the galaxy and after a huge war went into stasis, now in the present timeline of the 40k universe they are starting to wake back up. Only the Lords and Overlords (the leaders) survived the process with their minds relatively intact and went through the Robotization process willingly, then had their soldiers (who also went through the robotization process) round up the regular people and forced them into the machines, (they are very similar to Dr. Who Cybermen and I would be willing to bet heavily inspired by them, however, they don't Assimilate random people (like humans and other aliens) they just murder them with ancient yet hyper-advanced tech almost indistinguishable from magic... as an example their Snipers/Assassins The Deathmarks, use interdimensional phase shifters to be able to get close to their targets...