The Necrons are an ancient xenos race that have existed in secret until recent history. They first emerged during the War in Heaven, where they created the C'tan and gave up their mortal bodies for a soulless machine existence. After the war, they disappeared into their tomb worlds and waited for the galaxy to repopulate itself; slowly they are reawakening and rebuilding their ancient empire.
The Necrons are inorganic and therefore cannot make use of the Warp; however, they possess the most advanced technology in the galaxy, and have the only known FTL technology that doesn't use the Warp. They are also extremely hard to kill as they use "reanimation protocols" to revive fallen troops.
While many of the C'Tan were destroyed or eaten during the War in Heaven, those that survived were broken into shards; while each shard lacks the full power of the original, they are not powerless and are fully capable of operating independently.