The Aeldari had not been content to merely play the role of spectators in the ensuing drama and had secretly brought a large army to Messen. As the Abaddon’s Chaos forces deployed on the planet, using an old imperial outpost, the Aeldari struck. Led by Eldrad Ulthran himself as well as several Phoenix Lords, a combined force from several Craftworlds made an attempt to drive the Chaos forces away. Ulthran had foreseen the possible destruction of Messen and the closest Craftworlds if Abaddon was allowed to continue unhindered. After a hard struggle the noble Aeldari were defeated at the hands of Abaddon himself.
With reinforcements still coming in from the Webway gate, Abaddon’s forces had established a secure beachhead on Messen. As the Iron Warriors deployed on the planet and began the construction of several forts to acts as strongpoints when the imperial troops would arrive.