The most Hubward systems within the Mabb Nebula had been off limits to all human craft for millenia. Most saw "celestial hazard" printed on their star-charts but those in high places, or at least in privileged places, knew there existed at least five Eldar exodite worlds as well as rumours of a Craftworld. Additionally the Eldar laid claim to several uninhabited "maiden worlds". For thousands of years humanity had known to leave well alone.
Greenskins however have no respect and apparently short memories. In 673M37 an Ork Waagh! originating from the Vork Ork Empire had ravaged the worlds of the Mabb Nebula before disappearing mysteriously. Most thought the Eldar had lured the brutal aliens into a trap, and annihilated them. On 2606.008M42 the appearance of Orks on Memnon was relayed to Imperial HQ. Two days later the Eldar arrived.
Memnon has always been as close as humanity has been allowed to get to the Eldar holdings in the Mabb Nebula, and clearly the appearance of Orks on the world was enough to stir the graceful aliens into action and they soon threw themselves into a savage battle against the Greenskin horde. This time however the Orks had the measure of the Eldar and punished them severely. After a short, savage engagement the Eldar withdrew, leaving the Orks braying in wild celebration.