The Grey Raven (The Horus Heresy #Short Story)

Though the Imperium defends its worlds with vigour, not all cries for help are answered in time, or even heard at all. A nameless Imperial world has been conquered by orks; its resources seized for the greenskins’ war effort, and its human population enslaved. The eldar have foreseen that if the ork warlord is not stopped, and soon, the consequences will be dire. Enraged by its losses, the Imperium will eventually launch an anti-xenos crusade of unparalleled ferocity. To protect the future of their own craftworld, the eldar must be prepared to sacrifice all to defeat the ork invaders.

Read it because
Those tricksy eldar are up to their usual ploys – manipulating mankind for their own benefit. But with humans on one side and orks on the other, have the aliens bitten off more than they can chew this time?