The planet which the Blood Angel Scouts arrived upon was a giant volcanic jungle world, called Messen. It is located on the northern border of Segmentum Solar, in the vicinity to the Eye of Terror. The Imperium uses the planet to collect energy from the numerous thermal power plants established there during the Tagean Crusades. These supply power to the almost fully automated factories there to produce power cells used in various imperial energy weapons.
On the other side of the planet, a freshly recruited Dark Angel force was training. Their base of operations was one of the remote thermal plants. Suddenly, the training became real as a Drukhari force, led by Archaon Kael, appeared. After dispatching the guards, the Drukhari force easily ambushed and destroyed the arriving Dark Angels one squad at a time. The few that survived were taken as captives and set to work on rerouting the power plants energy output to power up an old deserted Webway gate. But the portal remained closed. The Haemonculi employed by the Cabal of Crimson Carnage stated that they needed more power for it to open and so Kael left him in charge as he took a contingent of troops to find another.
In the southern jungle he found what he was looking for, but as he approached it, he spotted another force making its way towards it as well. It was an Aeldari force from the Ulthwé Craftworld. Unknown to the Imperials, Messen was originally an Aeldari maiden world populated by Exodites. As long as the Imperials were content to gather energy from the thermal power plants and otherwise leaving the planet alone, the Exodites made no fuss. But as soon as the Drukhari presence was known, the strongest Exodite Lord, Isdale, had requested help from the nearest Craftworld. Eldrad Ulthran had consulted the runes and found that if Kael was not stopped, Ulthwé could be in danger. After a bloody battle, the Drukhari withdrew and the victorious Aeldari held the badly damaged power plant.