OOC: For me its step 1: Intro.
Everyone who goes will meet Eldrad, he'll be cryptic as fuck, before showing them a world the Tyranids have already consumed to give them an example, then warning that a splinter is heading for them but they can stop it, if they do as he says. First mission: Acquire gene-sample of a creature with a high connection to the Hive Mind. It boils down to the group entering a cave system and a possibility of: Massively delaying the Fleet and acquiring a large gene-sample, Disorganising the Tyranids on the planet for awhile giving the few defenders left more of a chance to wear the swarm down and acquiring a moderate gene-sample, or simply throwing the 'nids in the tunnels into brief confusion and acquiring a lesser sample.