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.

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.