Sporocyst
Pretty much a immobile version of the Tyrannocyte that is launched with an even more powerful muscle spasms from the Hive Ships in order to bury and root itself into the ground like Ticks do into flesh. When burrowed, they release bacteria that pollute the atmosphere (the same way as Spore Chimneys do), and when threatened, it will roll out its five Deathspitters and vomit out Spore Mines or, in the case of Flyers, a Mucolid Spore. Added that the darn thing is a psychic resonator that strengthens the overall capabilities of Nids around it.
Short: It's both planetary digestion unit, strongpoint and psychic relay for the Tyranids. All put in one package.