Tyranids cross the galaxies and massive voids in between in vaste Hive Fleets of millions of sentient ships. Of the 3 major Hive Fleet invasions, two come from the Eastern fringe, but one is moving up from below the galactic plane. The ships contain everything required to sustain the fleet; some ships being purely for crushing resistance, the larger ships contain birthing pools to create the huge land armies of the Tyranids and even more ships when the need arises. This nutrients are replenished by scouring every planet conquered of every shred of biological matter, including all the Tyranids it seeded to the planet. Because the Tyranid forces are re-cycled in this way the "design" of the Tyranid warriors can be altered by genetic engineering, allowing the Tyranid forces to rapidly adapt to overcome any enemy. Further, the Tyranids absorb the DNA of defeated enemies, allowing for further genetic diversity. It is believed the Genestealer and Broodlord are derived from human DNA, the Zoanthrope from Eldar and the Biovore from Ork DNA.
By the nature of the hive fleets, fierce resistance on a planet will only make the fleet stronger in the end; for this reason only a few tactics have been found to work. The first is to draw the Tyranids into a large ground battle, forceing the fleet to deploy as large a ground force as possible. Once this is achieved, evacuate the planet as much as possible and perform Exterminatus. This means that the Hive Fleet will have lost all of the bio-material in the ground forces and by the nature of Exterminatus will not be able to recover any from the planet. This tactic is very successful, but at the cost of destroying a habitable planet every time, it is not a tactic that can be counted upon. It has been preojected that there may not even be enough habitable planets in the galaxy to stop the tyranid threat, especially if more Tyranid fleets arrive from outside the galaxy.
This process usually occurs by:
1) Insinuating Genestealers in the population. This step isn't always neccesarily present, but Genestealer Cults are frequent enough to warrant mention.
2) Mycetic Spores are launched from the Hive Ships, and Lictors and other Tyranids, such as the Genestealers, begin the attack.
3) The Hive Fleet moves into close orbit, haveing defeated local resistance in space.
4) The Tyranid Forces land in full, and begin the attack.
5) The slain bodies of the planet are thrown together into digestion pools, where capillary towers are lowered from the Hive Ships to gather nutrients.
6) All forms of life are extinguished, all nutrients are gathered, and the Hive Fleet leaves the empty husk of the world for another.
