hanks, I had it all wrong.thr Dragon is one of the surviving C’tan. The Silent King was the Necron who made the deal with the C’tan and oversaw the biotransferrance process. He also led the rebellion against the C’tan and then ordered the Necrons to sleep for 60million years whilst he went into the void between galaxies as self imposed exile for what he has done.
Previous lore has him returning because of the Tyranids threat - his plan for the Necrons is to take over an existing species to regain morta flesh (and a soul), and the Tyranids would prevent that. This piece of lore seems to directly contradict that, as it implies he made the Tyranids deliberately to wipe out all organic life.