Sporocyst
Pretty much a immobile version of the Tyrannocyte that is launched with 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 (or Barbed Stranglers/Venom Cannons if you don't mind paying extra) 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, pretty much paints it as a priority target if there are not more dangerous Tyranid Monstrous Creatures around.
Short: It's both planetary digestion unit, strongpoint and psychic relay for the Tyranids. All put in one package.
On The Tabletop
Sporocyst has a relatively decent statline of WS 2, BS 2, S 5, T 5, W 6, I 3, A 3, Ld 8, Sv 4+. Of course its low weapons skill and ballistic skill makes it a poor unit in combat, so it is best to place the Sporocyst at the backline and act as a supportive unit instead. Sporocyst have the special rules of Fearless and Infiltrate as well as having rules such as Instinctive Fire which allows each weapon on this model to automatically fire at the nearest enemy unit within range and line of sight. The shots are resolved at the end of the Shooting phase before Morale checks are taken. It also has the Immobile Pod, Psychic Resonator and Spore Node rules.
Sporocyst comes armed with five deathspitters but can be replaced with either a barbed strangler or venom cannons for 25 pts, depending if you want your Sporocyst to be good at infantry or vehicles, although it is still not recommended for the Sporocyst to fight on its own as it will be demolished if left unattended.