There was a recent WD that had a Necron Q&A. The Spyder creates a swarm at the start of the assault phase for the Necron's turn. It does not appear that there are any other restictions with the Spyder later assauting that turn. I imagine if it were also engaged in HTH from a previous turn, at the start of the Necron's assualt phase it could generate a swarm.
A forum community dedicated to Wargamers, Wargaming 40K enthusiasts. Come join the discussion about Warhammer and Wargaming collections, miniatures, tactics, terrain, reviews, accessories, history, displays, models, styles, scales, reviews, accessories, classifieds, and more!