About halfway through 752.M41, somewhere in the northern edge of Segmentum Obscurus, WAAAGH! Grognak was on the warpath. Though by no means small - no Ork WAAAGH! ever is - the force led by the brutally kunnin Warboss Grognak was smaller than one would expect for as many planets he'd conquered. This was mostly due to the majority of his forces remaining in orbit; they were needed to crew the massive fleet of ork warships that flew his bannahs. Reports are sketchy, but it is believed that at the peak of his WAAAGH!, his fleet contained no less than 3 Space Hulks, and over 20 Kroozer-weight ships; this number does not take into account the number of escort craft, support ships, and roks accompanying the heavy hitters in the fleet.
Despite the Imperial Guard reinforcements mobilizing on the planet piecemeal - due to unfavorable warp currents, it is believed that the fleet's arrival in system was spread out to a single cruiser every few days - the WAAAGH! met it's end on planet [[PLANETARY NAME OMITTED: BY ORDER OF THE INQUISITION]]. Of the 15 regiments initially sent to deal with the Ork invasion, less than 20% of all fighting personnel and less than 1% of command personnel survived planetfall. The survivors, valiant and courageous soldiers of the Immortal God-Emperor to a man, rallied and initiated a guerrilla campaign against the Orks. This stymied the momentum of the WAAAGH! long enough for the 2nd wave of reinforcements to arrive - this time spearheaded by a Black Templars crusade fleet.
It is not clear as to the fate of Warboss Grognak. It is believed that he was killed in the initial assault when the Black Templars Space Marines made planetfall, though some scattered reports make mention of his death at the hands of the guardsmen responsible for the guerrilla resistance well before the arrival of the Black Templars in system. As is the case with so many war reports - especially so with a war carried out in such a fragmented way as this - it will likely be impossible to ever be certain of the truth.