Hive Fleet Kraken
Hive Fleet Kraken was the second Hive fleet of the Tyranids to assault the 40k galaxy and started the Second Tyrannic War which was only stopped when the Ultramarines and Eldar of Craftworld Iyanden defeated the two major prongs of the invasion, although at great cost and not at the destruction of all the splinter fleets of Kraken, which continue to infest and ravage the Imperium to this day.
Foreboding of a horror
Kraken arrived two and a half centuries after the defeat of Hive Fleet Behemoth, when most people except the Inquisition (who are just too paranoid to let their guard down) had been lulled into a false sense of relief that at least the bugs were dead. How foolish that now seems; thank the Emperor the Inquisition were too suspicious to be fooled as well.
After some intense investigating, they discovered there was strange activity coming from the south east corner of the galaxy, a spate of rebellions and the like which caused great unrest; rebellion is nothing new in the Imperium but these people were doing it more regularly than most. Matters came to a head when Ichar IV, a vital hive-world in the galactic south, flared up into revolt and it was discovered that the cause was not chaotic in nature but xenoic. Tyranid genestealers led by a broodlord, having tainted countless human slaves, formed a psychic beacon, shining out into the dark regions beyond the galaxy. A space marine terminator strike force attacked, but it was too late, and the death scream of the broodlord mixed with a signal that called out to its masters. From the depths of space, Hive Fleet Kraken sped towards the Imperium.
Arrival and two legendary struggles
Within weeks the hive fleet of Kraken arrived on Imperial borders and all hell broke loose. Worlds and their peoples were consumed at a vicious rate and the Tyranids ploughed straight into the Imperium, attacking over a wide range. Eventually though, they combined their smaller forces into two larger prongs and struck Ichar IV and the Eldar craftworld of Iyanden.
At Ichar IV the Ultramarines and the rest of the Imperial defenders (plus some Eldar who turned up to join in) managed to crush the Tyranid advance although it left the planet a smoking ruin. The fluff implies it was the sheer mary-suedom of the Ultramarines that won the day, overwhelming the 'nids through their efforts mostly.
Meanwhile at Iyanden the Tyranids had overtaken the slower craftworld and the Eldar became engulfed in a battle for their home, one which it seemed they were doomed to lose. Only the aid of Prince Yriel and his Eldritch Raiders saved the craftworld and even then the cost was horrific; four-fifths of the Eldar of Iyanden lay dead and the craftworld was shattered, a shell of its former self.
A threat not vanquished
It was these two fierce struggles that saw the defeat of the bulk of Kraken's forces, although perhaps it's true threat had not yet been realized. Many splinter fleets of Kraken escaped destruction into the depths of Imperial space and attacked defenseless worlds, becoming huge and bloated and a threat in their own right. These splinter fleets continue to attack the Imperium from within to this day, draining valuable resources and weakening the prey…
A tendril of the Kraken fleet also made its way to Vior'los, the largest planet in the Farsight Enclaves. This was the event that got the commander to come out of retirement and form the 8. They held the line against the beasts while the Earth Castes developed a toxin strong enough to overcome the Tyranid immune system. Once it was finished they set to a timed release so that it would activate once all the biomass was lumped together, that way they would all die at the same time and nothing would be left to become immune. Once again, the Farsight Enclaves get shit done.
The Eldar aren't done suffering either, if the upcoming Warzone Valedor is any indication. Indeed, when Kraken's survivors emerged over the planet of Duriel (as the Eldar called it) of the Valedor system, they found Hive Fleet Leviathan already there eating up the delicious Imperial hives. The Iyanden farseers had already forseen this and knew how dangerous two hive fleets eating each other could be, and rallied all the Eldar they could ahead of time to intercept the events. Only one craftworld could respond in time: Biel-Tan. Since this wasn't nearly enough to fight off two hive fleets, they also had to strike a bargain with the Dark Eldar to turn the tide.
Also, in a pattern of being fucked over by everything and everyone, the Lamenters just happened to bump into another of Kraken's splinter fleets by fucking chance. Just as planned. As of recent the Lamenters have less than 400 marines to combat the Nids, but they'll fight to the end like the goddamn badasses they are.