Szarekh came back because Tyranids

• Shield of Baal [Word of the Silent King]:

"Long have we known of the Devourer. While the majority of the necron race slept away the aeons, his great majesty Szarekh, the Silent King, journeyed far and wide beyond the borders of this galaxy. Such unspeakable things did he witness as cannot be adequately articulated in our noble language, nor any other.

The most dire of all these extragalactic enemies were the tyranids.

For countless cycles he has sought to repel this threat. In his wisdom he has observed them, studied them and committed them to oblivion in all but the final, decisive deed. He has brought them to battle on a hundred worlds, ravaged their slumbering fleets out in the cold, measureless void, and even united the more fractious, warring dynasties so that our mutual interests might be protected."