Two and a half centuries had passed since the the first Tyranid fleet had attacked the imperium. Some of the Adeptus Administratums started to argue that it's a waste to have all thousands of armoured troops and ships awaiting a Tyranid threat which didn't exist. That all changed at the rebellion on Ichar IV.
The rebellion was quick and bloody. The rebellion had started by a fundamentalist and religious cult called the Brotherhood. The preached to the lower class about the return of the emperor. Thousands followed with total devotion. The Brotherhood was investigated for heresy bot no evidence was found. The brotherhood was also poermitted to build a cathedral in Lomas which was the biggest city on the planet. After finishing construction of the cathedral was when the trouble started.
The brotherhood wouldn't pay their tithes to the govenor or let any of its members be part of the planetary defence force. Then the Brotherhood whipped the population into a frenzy and claimed the emperor would arrive soon. There were riots in the streets and the guards were overrun and when the planetary defence force was sent in most turned on the government, and amoungnst all this the governor was assinated.
The battle was a stalemate for until the space marines took over the governer's house which was the HQ for the brotherhood and many planetary garrisons. The Brotherhood's counter attack were desperate and sloppy and cost them many men. When Ultramarine terminators went to take over the cathedral and took out most of the Brotherhood but a few jumped them wtih inhuman strength and speed but soon taken out. The terminators went down into a labryinth of caves where they killed genestealers and a patriarch. After the battle there were many question like where was the Genestealer magus who lead the Brotherhood?
After the rebellion people started to flee from the outer world planets and a disturbance in the warp made communication impossible with outer fringed worlds and warp travel extremely dangerous. There was a new hive fleet called Kraken, but they were different from Behemoth because Kraken attacked in sub fleets taking over many planets at once while Behemoth took over one planet at a time. The description of the attacks from survivors who were hundreds of years off course described the skies of whole continents went black from falling pods and major city centres destroyed in a night. Some people were taken alive so be fed upon and they envied the dead.
This force still approaching ever closer towards earth. The Tyranids seem to feed upon their victims to evolve new creatures and adapt to their new surroundings. Now the Tyranids are in the galaxy of the imperium erver changing add able to keep feeding for a very long time on the billions of people within the galaxy.